Unicast link radio link failure detection and management

ABSTRACT

Methods, systems, and devices for wireless communications are described. A first user equipment (UE) may establish a sidelink connection with a second UE, where the sidelink connection is associated with a plurality of flows. The first UE may determine, based on monitoring each flow of the plurality of flows, a radio link status of the sidelink connection. The first UE may transmit, based on the determining, a non-access stratum layer message to the second UE based at least on the radio link status of the sidelink connection.

CROSS REFERENCE

The present application for patent claims the benefit of U.S.Provisional Patent Application No. 62/951,771 by Cheng et al., entitled“UNICAST LINK RADIO LINK FAILURE DETECTION AND MANAGEMENT,” filed Dec.20, 2019, assigned to the assignee hereof, and expressly incorporated byreference in its entirety herein.

BACKGROUND

The following relates generally to wireless communications, and morespecifically to unicast link radio link failure (RLF) detection andmanagement.

Wireless communications systems are widely deployed to provide varioustypes of communication content such as voice, video, packet data,messaging, broadcast, and so on. These systems may be capable ofsupporting communication with multiple users by sharing the availablesystem resources (e.g., time, frequency, and power). Examples of suchmultiple-access systems include fourth generation (4G) systems such asLong Term Evolution (LTE) systems, LTE-Advanced (LTE-A) systems, orLTE-A Pro systems, and fifth generation (5G) systems which may bereferred to as New Radio (NR) systems. These systems may employtechnologies such as code division multiple access (CDMA), time divisionmultiple access (TDMA), frequency division multiple access (FDMA),orthogonal frequency division multiple access (OFDMA), or discreteFourier transform spread orthogonal frequency division multiplexing(DFT-S-OFDM). A wireless multiple-access communications system mayinclude a number of base stations or network access nodes, eachsimultaneously supporting communication for multiple communicationdevices, which may be otherwise known as user equipment (UE).

SUMMARY

The described techniques relate to improved methods, systems, devices,and apparatuses that support unicast link radio link failure (RLF)detection and management. Generally, the described techniques providevarious mechanisms that support wireless communications in a wirelessnetwork. Broadly, aspects of the described techniques implement radiolink management (RLM)/radio link failure (RLF) detection over a sidelinkchannel. For example, two user equipment (UE) may be communicating overa sidelink channel. The sidelink connection may include a plurality offlows being used for data communications between the UEs. A flow broadlyrefers to data communicated over separate data radio bearers (DRBs),having different quality-of-service (QoS) requirements, and the like.Each UE may respectively monitor communications over each flow in orderto determine the radio link status of the sidelink connections. Forexample, each UE may monitor for acknowledgment messages associated withcommunications performed over each flow, may monitor the amount of databeing communicated over each flow according to a time period, orotherwise determine whether or not the data is being communicated acrosseach flow in an acceptable manner and satisfying an associated QoSrequirement. The UEs may exchange non-access stratum (NAS) layermessages and/or access stratum (AS) messages based on the radio linkstatus of the sidelink connection. For example, a UE may determine thatat least one of the flows has no data being communicated, e.g., based ona lack of acknowledgment messages, based on an inactivity timer, basedon failing to satisfy the QoS requirement for the flow, and the like.

Generally, the lack of data being communicated over a flow may trigger aconcern that the sidelink channel has degraded to below an acceptableperformance level and/or has dropped (e.g., is experiencing RLF).Accordingly, rather than simply declaring a RLF, the UE may determinewhether data is being communicated across the other flow(s) of thesidelink connection. If data is being communicated over at least oneother flow of the sidelink connection, this may indicate that thesidelink connection has not failed, but may have degraded to a degreeand therefore various parameters for the sidelink channel may beupdated. In this situation, the UE may transmit a message (e.g., a NASmessage and/or an AS message) to the other UE updating or otherwisereconfiguring parameters for the sidelink connection, e.g., updating theQoS parameters, selecting a new configuration for the sidelinkconnection, etc. If the UE determines that no data is being communicatedacross the other flow(s), this may indicate that the sidelink connectionhas failed and/or more significantly degraded. In this situation, the UEmay transmit a keep alive message to the other UE over the sidelinkchannel (e.g., using a signaling radio bearer (SRB) and/or one or moreDRBs of the sidelink connection) requesting confirmation that thesidelink connection is active. If a response to the keep alive messageindicates that the sidelink connection is active, the UE may dismiss thelack of communications on the particular flow and/or transmit a message(e.g., a NAS and/or AS message) to the other UE reconfiguring variousparameters for the sidelink connection. If no response to the keep alivemessage is received from the other UE, the UE may determine that thesidelink connection has failed, and therefore teardown that connectionand begin an RLF recovery procedure. These techniques improve linkmanagement for the sidelink connection between UEs, avoid unnecessaryRLF declaration (e.g., unnecessarily tearing down the sidelinkconnection and having to rebuild a new sidelink connection), and thelike.

A method of wireless communication at a first UE is described. Themethod may include establishing a sidelink connection with a second UE,where the sidelink connection is associated with a set of flows,determining, based on monitoring each flow of the set of flows, a radiolink status of the sidelink connection, and transmitting, based on thedetermining, a non-access stratum layer message to the second UE basedat least on the radio link status of the sidelink connection.

An apparatus for wireless communication at a first UE is described. Theapparatus may include a processor, memory coupled with the processor,and instructions stored in the memory. The instructions may beexecutable by the processor to cause the apparatus to establish asidelink connection with a second UE, where the sidelink connection isassociated with a set of flows, determine, based on monitoring each flowof the set of flows, a radio link status of the sidelink connection, andtransmit, based on the determining, a non-access stratum layer messageto the second UE based at least on the radio link status of the sidelinkconnection.

Another apparatus for wireless communication at a first UE is described.The apparatus may include means for establishing a sidelink connectionwith a second UE, where the sidelink connection is associated with a setof flows, determining, based on monitoring each flow of the set offlows, a radio link status of the sidelink connection, and transmitting,based on the determining, a non-access stratum layer message to thesecond UE based at least on the radio link status of the sidelinkconnection.

A non-transitory computer-readable medium storing code for wirelesscommunication at a first UE is described. The code may includeinstructions executable by a processor to establish a sidelinkconnection with a second UE, where the sidelink connection is associatedwith a set of flows, determine, based on monitoring each flow of the setof flows, a radio link status of the sidelink connection, and transmit,based on the determining, a non-access stratum layer message to thesecond UE based at least on the radio link status of the sidelinkconnection.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for determining that nodata may be communicated across a first flow of the set of flows, andtransmitting the non-access stratum layer message to the second UE,where the non-access stratum layer message includes a keep-alive messagerequesting confirmation from the second UE that the radio link status ofthe sidelink connection may be active.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for receiving a responsemessage from the second UE indicating that the radio link status of thesidelink connection with the second UE may be active, and transmitting,based on the response message, a second message to the second UEreconfiguring one or more parameters of the sidelink connection.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, reconfiguring one or moreparameters of the sidelink connection may include operations, features,means, or instructions for reconfiguring the sidelink connection from afirst configuration to a second configuration from a set of availableconfigurations configured for the sidelink connection.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, reconfiguring one or moreparameters of the sidelink connection may include operations, features,means, or instructions for reconfiguring one or more quality of serviceparameters configured for the sidelink connection.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for determining, based on alack of a response message from the second UE, that the radio linkstatus of the sidelink connection with the second UE includes a radiolink failure, and performing, based on the radio link failure, a radiolink failure recovery procedure to establish a second sidelinkconnection with the second UE.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, determining that no data maybe communicated may include operations, features, means, or instructionsfor determining that data communicated across the first flow fails tosatisfy a quality of service requirement associated with the first flow.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, determining that no data maybe communicated may include operations, features, means, or instructionsfor determining that no data may have been communicated across the firstflow for a threshold time period associated with the first flow.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for determining that nodata may be communicated across a first flow of the set of flows,determining that data may be communicated across a second flow of theset of flows, determining that the radio link status of the sidelinkconnection includes a degraded radio link status, and transmitting asecond message to the second UE reconfiguring one or more parameters ofthe sidelink connection.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, reconfiguring one or moreparameters of the sidelink connection may include operations, features,means, or instructions for reconfiguring the sidelink connection from afirst configuration to a second configuration from a set of availableconfigurations configured for the sidelink connection.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, reconfiguring one or moreparameters of the sidelink connection may include operations, features,means, or instructions for reconfiguring one or more quality of serviceparameters configured for the sidelink connection.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for determining that thenon-access stratum layer message was transmitted within a threshold timeperiod, and refraining from transmitting a second non-access stratumlayer message to the second UE based at least on the non-access stratumlayer message.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, determining the radio linkstatus of the sidelink connection may include operations, features,means, or instructions for determining, for each flow of the set offlows, whether data communicated across each flow satisfies a quality ofservice requirement configured for the flow.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the non-access stratum layermessage includes a PC5 sidelink (PC5-S) message.

A method of wireless communication at a second UE is described. Themethod may include establishing a sidelink connection with a first UE,where the sidelink connection is associated with a set of flows,receiving a non-access stratum layer message from the first UEindicating a radio link status of the sidelink connection, where thenon-access stratum layer message is based on a status at the first UE ofeach flow of the set of flows, and transmitting a response message tothe first UE indicating that the radio link status of the sidelinkconnection with the first UE is active.

An apparatus for wireless communication at a second UE is described. Theapparatus may include a processor, memory coupled with the processor,and instructions stored in the memory. The instructions may beexecutable by the processor to cause the apparatus to establish asidelink connection with a first UE, where the sidelink connection isassociated with a set of flows, receive a non-access stratum layermessage from the first UE indicating a radio link status of the sidelinkconnection, where the non-access stratum layer message is based on astatus at the first UE of each flow of the set of flows, and transmit aresponse message to the first UE indicating that the radio link statusof the sidelink connection with the first UE is active.

Another apparatus for wireless communication at a second UE isdescribed. The apparatus may include means for establishing a sidelinkconnection with a first UE, where the sidelink connection is associatedwith a set of flows, receiving a non-access stratum layer message fromthe first UE indicating a radio link status of the sidelink connection,where the non-access stratum layer message is based on a status at thefirst UE of each flow of the set of flows, and transmitting a responsemessage to the first UE indicating that the radio link status of thesidelink connection with the first UE is active.

A non-transitory computer-readable medium storing code for wirelesscommunication at a second UE is described. The code may includeinstructions executable by a processor to establish a sidelinkconnection with a first UE, where the sidelink connection is associatedwith a set of flows, receive a non-access stratum layer message from thefirst UE indicating a radio link status of the sidelink connection,where the non-access stratum layer message is based on a status at thefirst UE of each flow of the set of flows, and transmit a responsemessage to the first UE indicating that the radio link status of thesidelink connection with the first UE is active.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for receiving, based on theresponse message, a second message from the first UE reconfiguring oneor more parameters of the sidelink connection, and reconfiguring the oneor more parameters of the sidelink connection based at least on part onthe second message.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, reconfiguring one or moreparameters of the sidelink connection may include operations, features,means, or instructions for reconfiguring the sidelink connection from afirst configuration to a second configuration from a set of availableconfigurations configured for the sidelink connection.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, reconfiguring one or moreparameters of the sidelink connection may include operations, features,means, or instructions for reconfiguring one or more quality of serviceparameters configured for the sidelink connection.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the non-access stratum layermessage includes a keep-alive message requesting confirmation from thesecond UE that the radio link status of the sidelink connection may beactive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a system for wireless communicationsthat supports unicast link radio link failure (RLF) detection andmanagement in accordance with aspects of the present disclosure.

FIG. 2 illustrates an example of a wireless communication system thatsupports unicast link RLF detection and management in accordance withaspects of the present disclosure.

FIG. 3 illustrates an example of a process that supports unicast linkRLF detection and management in accordance with aspects of the presentdisclosure.

FIG. 4 illustrates an example of a process that supports unicast linkRLF detection and management in accordance with aspects of the presentdisclosure.

FIG. 5 illustrates an example of a process that supports unicast linkRLF detection and management in accordance with aspects of the presentdisclosure.

FIGS. 6 and 7 show block diagrams of devices that support unicast linkRLF detection and management in accordance with aspects of the presentdisclosure.

FIG. 8 shows a block diagram of a communications manager that supportsunicast link RLF detection and management in accordance with aspects ofthe present disclosure.

FIG. 9 shows a diagram of a system including a device that supportsunicast link RLF detection and management in accordance with aspects ofthe present disclosure.

FIGS. 10 through 14 show flowcharts illustrating methods that supportunicast link RLF detection and management in accordance with aspects ofthe present disclosure.

DETAILED DESCRIPTION

Wireless communication systems may use different interfaces to supportwireless communications between devices. For example, a Uu interface maybe used to support wireless communications between a base station and auser equipment (UE). A PC5 interface may be used to support wirelesscommunications between UEs. Each interface type is unique in terms ofconfigurations, requirements, etc. For example, two UEs may establish asidelink connection over a PC5 interface, with the sidelink connectionsupporting multiple flows. A flow broadly refers to data communicatedover separate data radio bearers (DRBs), data communicated havingdifferent quality-of-service (QoS) requirements, and the like. Somewireless communication systems are configured such that no accessstratum (AS) layer signaling can be used for radio link management(RLM)/radio link failure (RLF). Moreover, such wireless communicationsystems may also be configured such that there is no receiver side lowerlayer indication supported for RLF. Accordingly, this may result in UEscommunicating over a sidelink channel having no effective or efficientmeans of managing the sidelink connection. In some aspects, this maymean that the UEs unnecessarily declare RLF for the sidelink connectionin the situation where the link is still active, but may have degradedto some degree. Unnecessarily declaring RLF means that considerableresources and time are wasted tearing down the sidelink connection andthen rebuilding a new sidelink connection.

Aspects of the disclosure are initially described in the context of awireless communication system. Broadly, aspects of the describedtechniques implement RLM/RLF detection over a sidelink channel. Forexample, two UE may be communicating over a sidelink channel. Thesidelink connection may include a plurality of flows being used for datacommunications between the UEs. A flow broadly refers to datacommunicated over separate DRBs, having different QoS requirements, andthe like. Each UE may respectively monitor communications over each flowin order to determine the radio link status of the sidelink connections.For example, each UE may monitor for acknowledgment messages associatedwith communications performed over each flow, may monitor the amount ofdata being communicated over each flow according to a time period, orotherwise may determine whether or not the data is being communicatedacross each flow in an acceptable manner and satisfying an associatedQoS requirement. The UEs may exchange NAS layer messages and/or ASmessages based on the radio link status of the sidelink connection. Forexample, a UE may determine that at least one of the flows has no databeing communicated, e.g., based on a lack of acknowledgment messages,based on an inactivity timer, based on failing to satisfy the QoSrequirement for the flow, and the like.

Generally, the lack of data being communicated over a flow may trigger aconcern that the sidelink channel has degraded to below an acceptableperformance level and/or has dropped (e.g., is experiencing RLF).Accordingly and rather than simply declaring a RLF, the UE may determinewhether data is being communicated across the other flow(s) of thesidelink connection. If data is being communicated, this may indicatethat the sidelink connection may have degraded to a degree and thereforevarious parameters for the sidelink channel may be updated. In thissituation, the UE may transmit a message (e.g., a NAS message and/or anAS message) to the other UE updating or otherwise reconfiguringparameters for the sidelink connection, e.g., updating the QoSparameters, selecting a new configuration for the sidelink connection,etc. In other cases, if data is being communicated across other flow(s)of the sidelink connection, the link may be operating properly and thealarm may be due to lack of application layer data to transmit for thisparticular flow. If the UE determines that no data is being communicatedacross the other flow(s), this may indicate that the sidelink connectionhas failed and/or more significantly degraded. In this situation, the UEmay transmit a keep alive message to the other UE over the sidelinkchannel (e.g., using a signaling radio bearer (SRB) and/or one or moreDRBs of the sidelink connection) requesting confirmation that thesidelink connection is active. If a response to the keep alive messageindicates that the sidelink connection is active, the UE may dismiss thelack of communications on the particular flow and/or transmit a message(e.g., a NAS and/or AS message) to the other UE reconfiguring variousparameters for the sidelink connection. If no response to the keep alivemessage is received from the other UE, the UE may determine that thesidelink connection has failed, and therefore teardown that connectionand begin an RLF recovery procedure. These techniques improve linkmanagement for the sidelink connection between UEs, avoid unnecessaryRLF declaration (e.g., unnecessarily tearing down the sidelinkconnection and having to rebuild a new sidelink connection), and thelike.

Aspects of the disclosure are further illustrated by and described withreference to apparatus diagrams, system diagrams, and flowcharts thatrelate to unicast link RLF detection and management.

FIG. 1 illustrates an example of a wireless communication system 100that supports unicast link RLF detection and management in accordancewith aspects of the present disclosure. The wireless communicationsystem 100 includes base stations 105, UEs 115, and a core network 130.In some examples, the wireless communication system 100 may be a LongTerm Evolution (LTE) network, an LTE-Advanced (LTE-A) network, an LTE-APro network, or a New Radio (NR) network. In some cases, wirelesscommunication system 100 may support enhanced broadband communications,ultra-reliable (e.g., mission critical) communications, low latencycommunications, or communications with low-cost and low-complexitydevices.

Base stations 105 may wirelessly communicate with UEs 115 via one ormore base station antennas. Base stations 105 described herein mayinclude or may be referred to by those skilled in the art as a basetransceiver station, a radio base station, an access point, a radiotransceiver, a NodeB, an eNodeB (eNB), a next-generation NodeB orgiga-NodeB (either of which may be referred to as a gNB), a Home NodeB,a Home eNodeB, or some other suitable terminology. Wirelesscommunication system 100 may include base stations 105 of differenttypes (e.g., macro or small cell base stations). The UEs 115 describedherein may be able to communicate with various types of base stations105 and network equipment including macro eNBs, small cell eNBs, gNBs,relay base stations, and the like.

Each base station 105 may be associated with a particular geographiccoverage area 110 in which communications with various UEs 115 issupported. Each base station 105 may provide communication coverage fora respective geographic coverage area 110 via communication links 125,and communication links 125 between a base station 105 and a UE 115 mayutilize one or more carriers. Communication links 125 shown in wirelesscommunication system 100 may include uplink transmissions from a UE 115to a base station 105, or downlink transmissions from a base station 105to a UE 115. Downlink transmissions may also be called forward linktransmissions while uplink transmissions may also be called reverse linktransmissions.

The geographic coverage area 110 for a base station 105 may be dividedinto sectors making up a portion of the geographic coverage area 110,and each sector may be associated with a cell. For example, each basestation 105 may provide communication coverage for a macro cell, a smallcell, a hot spot, or other types of cells, or various combinationsthereof. In some examples, a base station 105 may be movable andtherefore provide communication coverage for a moving geographiccoverage area 110. In some examples, different geographic coverage areas110 associated with different technologies may overlap, and overlappinggeographic coverage areas 110 associated with different technologies maybe supported by the same base station 105 or by different base stations105. The wireless communication system 100 may include, for example, aheterogeneous LTE/LTE-A/LTE-A Pro or NR network in which different typesof base stations 105 provide coverage for various geographic coverageareas 110.

The term “cell” refers to a logical communication entity used forcommunication with a base station 105 (e.g., over a carrier), and may beassociated with an identifier for distinguishing neighboring cells(e.g., a physical cell identifier (PCID), a virtual cell identifier(VCID)) operating via the same or a different carrier. In some examples,a carrier may support multiple cells, and different cells may beconfigured according to different protocol types (e.g., machine-typecommunication (MTC), narrowband Internet-of-Things (NB-IoT), enhancedmobile broadband (eMBB), or others) that may provide access fordifferent types of devices. In some cases, the term “cell” may refer toa portion of a geographic coverage area 110 (e.g., a sector) over whichthe logical entity operates.

UEs 115 may be dispersed throughout the wireless communication system100, and each UE 115 may be stationary or mobile. A UE 115 may also bereferred to as a mobile device, a wireless device, a remote device, ahandheld device, or a subscriber device, or some other suitableterminology, where the “device” may also be referred to as a unit, astation, a terminal, or a client. A UE 115 may also be a personalelectronic device such as a cellular phone, a personal digital assistant(PDA), a tablet computer, a laptop computer, or a personal computer. Insome examples, a UE 115 may also refer to a wireless local loop (WLL)station, an Internet of Things (IoT) device, an Internet of Everything(IoE) device, or an MTC device, or the like, which may be implemented invarious articles such as appliances, vehicles, meters, or the like.

Some UEs 115, such as MTC or IoT devices, may be low cost or lowcomplexity devices, and may provide for automated communication betweenmachines (e.g., via Machine-to-Machine (M2M) communication). M2Mcommunication or MTC may refer to data communication technologies thatallow devices to communicate with one another or a base station 105without human intervention. In some examples, M2M communication or MTCmay include communications from devices that integrate sensors or metersto measure or capture information and relay that information to acentral server or application program that can make use of theinformation or present the information to humans interacting with theprogram or application. Some UEs 115 may be designed to collectinformation or enable automated behavior of machines. Examples ofapplications for MTC devices include smart metering, inventorymonitoring, water level monitoring, equipment monitoring, healthcaremonitoring, wildlife monitoring, weather and geological eventmonitoring, fleet management and tracking, remote security sensing,physical access control, and transaction-based business charging.

Some UEs 115 may be configured to employ operating modes that reducepower consumption, such as half-duplex communications (e.g., a mode thatsupports one-way communication via transmission or reception, but nottransmission and reception simultaneously). In some examples,half-duplex communications may be performed at a reduced peak rate.Other power conservation techniques for UEs 115 include entering a powersaving “deep sleep” mode when not engaging in active communications, oroperating over a limited bandwidth (e.g., according to narrowbandcommunications). In some cases, UEs 115 may be designed to supportcritical functions (e.g., mission critical functions), and a wirelesscommunication system 100 may be configured to provide ultra-reliablecommunications for these functions.

In some cases, a UE 115 may also be able to communicate directly withother UEs 115 (e.g., using a peer-to-peer (P2P) or device-to-device(D2D) protocol). One or more of a group of UEs 115 utilizing D2Dcommunications may be within the geographic coverage area 110 of a basestation 105. Other UEs 115 in such a group may be outside the geographiccoverage area 110 of a base station 105, or be otherwise unable toreceive transmissions from a base station 105. In some cases, groups ofUEs 115 communicating via D2D communications may utilize a one-to-many(1:M) system in which each UE 115 transmits to every other UE 115 in thegroup. In some cases, a base station 105 facilitates the scheduling ofresources for D2D communications. In other cases, D2D communications arecarried out between UEs 115 without the involvement of a base station105.

Base stations 105 may communicate with the core network 130 and with oneanother. For example, base stations 105 may interface with the corenetwork 130 through backhaul links 132 (e.g., via an S1, N2, N3, orother interface). Base stations 105 may communicate with one anotherover backhaul links 134 (e.g., via an X2, Xn, or other interface) eitherdirectly (e.g., directly between base stations 105) or indirectly (e.g.,via core network 130).

The core network 130 may provide user authentication, accessauthorization, tracking, Internet Protocol (IP) connectivity, and otheraccess, routing, or mobility functions. The core network 130 may be anevolved packet core (EPC), which may include at least one mobilitymanagement entity (MME), at least one serving gateway (S-GW), and atleast one Packet Data Network (PDN) gateway (P-GW). The MME may managenon-access stratum (e.g., control plane) functions such as mobility,authentication, and bearer management for UEs 115 served by basestations 105 associated with the EPC. User IP packets may be transferredthrough the S-GW, which itself may be connected to the P-GW. The P-GWmay provide IP address allocation as well as other functions. The P-GWmay be connected to the network operators IP services. The operators IPservices may include access to the Internet, Intranet(s), an IPMultimedia Subsystem (IMS), or a Packet-Switched (PS) Streaming Service.

At least some of the network devices, such as a base station 105, mayinclude subcomponents such as an access network entity, which may be anexample of an access node controller (ANC). Each access network entitymay communicate with UEs 115 through a number of other access networktransmission entities, which may be referred to as a radio head, a smartradio head, or a transmission/reception point (TRP). In someconfigurations, various functions of each access network entity or basestation 105 may be distributed across various network devices (e.g.,radio heads and access network controllers) or consolidated into asingle network device (e.g., a base station 105).

Wireless communication system 100 may operate using one or morefrequency bands, typically in the range of 300 megahertz (MHz) to 300gigahertz (GHz). Generally, the region from 300 MHz to 3 GHz is known asthe ultra-high frequency (UHF) region or decimeter band, since thewavelengths range from approximately one decimeter to one meter inlength. UHF waves may be blocked or redirected by buildings andenvironmental features. However, the waves may penetrate structuressufficiently for a macro cell to provide service to UEs 115 locatedindoors. Transmission of UHF waves may be associated with smallerantennas and shorter range (e.g., less than 100 km) compared totransmission using the smaller frequencies and longer waves of the highfrequency (HF) or very high frequency (VHF) portion of the spectrumbelow 300 MHz.

Wireless communication system 100 may also operate in a super highfrequency (SHF) region using frequency bands from 3 GHz to 30 GHz, alsoknown as the centimeter band. The SHF region includes bands such as the5 GHz industrial, scientific, and medical (ISM) bands, which may be usedopportunistically by devices that may be capable of toleratinginterference from other users.

Wireless communication system 100 may also operate in an extremely highfrequency (EHF) region of the spectrum (e.g., from 30 GHz to 300 GHz),also known as the millimeter band. In some examples, wirelesscommunication system 100 may support millimeter wave (mmW)communications between UEs 115 and base stations 105, and EHF antennasof the respective devices may be even smaller and more closely spacedthan UHF antennas. In some cases, this may facilitate use of antennaarrays within a UE 115. However, the propagation of EHF transmissionsmay be subject to even greater atmospheric attenuation and shorter rangethan SHF or UHF transmissions. Techniques disclosed herein may beemployed across transmissions that use one or more different frequencyregions, and designated use of bands across these frequency regions maydiffer by country or regulating body.

In some cases, wireless communication system 100 may utilize bothlicensed and unlicensed radio frequency spectrum bands. For example,wireless communication system 100 may employ License Assisted Access(LAA), LTE-Unlicensed (LTE-U) radio access technology, or NR technologyin an unlicensed band such as the 5 GHz ISM band. When operating inunlicensed radio frequency spectrum bands, wireless devices such as basestations 105 and UEs 115 may employ listen-before-talk (LBT) proceduresto ensure a frequency channel is clear before transmitting data. In somecases, operations in unlicensed bands may be based on a carrieraggregation configuration in conjunction with component carriersoperating in a licensed band (e.g., LAA). Operations in unlicensedspectrum may include downlink transmissions, uplink transmissions,peer-to-peer transmissions, or a combination of these. Duplexing inunlicensed spectrum may be based on frequency division duplexing (FDD),time division duplexing (TDD), or a combination of both.

In some examples, base station 105 or UE 115 may be equipped withmultiple antennas, which may be used to employ techniques such astransmit diversity, receive diversity, multiple-input multiple-output(MIMO) communications, or beamforming. For example, wirelesscommunication system 100 may use a transmission scheme between atransmitting device (e.g., a base station 105) and a receiving device(e.g., a UE 115), where the transmitting device is equipped withmultiple antennas and the receiving device is equipped with one or moreantennas. MIMO communications may employ multipath signal propagation toincrease the spectral efficiency by transmitting or receiving multiplesignals via different spatial layers, which may be referred to asspatial multiplexing. The multiple signals may, for example, betransmitted by the transmitting device via different antennas ordifferent combinations of antennas. Likewise, the multiple signals maybe received by the receiving device via different antennas or differentcombinations of antennas. Each of the multiple signals may be referredto as a separate spatial stream, and may carry bits associated with thesame data stream (e.g., the same codeword) or different data streams.Different spatial layers may be associated with different antenna portsused for channel measurement and reporting. MIMO techniques includesingle-user MIMO (SU-MIMO) where multiple spatial layers are transmittedto the same receiving device, and multiple-user MIMO (MU-MIMO) wheremultiple spatial layers are transmitted to multiple devices.

Beamforming, which may also be referred to as spatial filtering,directional transmission, or directional reception, is a signalprocessing technique that may be used at a transmitting device or areceiving device (e.g., a base station 105 or a UE 115) to shape orsteer an antenna beam (e.g., a transmit beam or receive beam) along aspatial path between the transmitting device and the receiving device.Beamforming may be achieved by combining the signals communicated viaantenna elements of an antenna array such that signals propagating atparticular orientations with respect to an antenna array experienceconstructive interference while others experience destructiveinterference. The adjustment of signals communicated via the antennaelements may include a transmitting device or a receiving deviceapplying certain amplitude and phase offsets to signals carried via eachof the antenna elements associated with the device. The adjustmentsassociated with each of the antenna elements may be defined by abeamforming weight set associated with a particular orientation (e.g.,with respect to the antenna array of the transmitting device orreceiving device, or with respect to some other orientation).

In one example, a base station 105 may use multiple antennas or antennaarrays to conduct beamforming operations for directional communicationswith a UE 115. For instance, some signals (e.g., synchronizationsignals, reference signals, beam selection signals, or other controlsignals) may be transmitted by a base station 105 multiple times indifferent directions, which may include a signal being transmittedaccording to different beamforming weight sets associated with differentdirections of transmission. Transmissions in different beam directionsmay be used to identify (e.g., by the base station 105 or a receivingdevice, such as a UE 115) a beam direction for subsequent transmissionand/or reception by the base station 105.

Some signals, such as data signals associated with a particularreceiving device, may be transmitted by a base station 105 in a singlebeam direction (e.g., a direction associated with the receiving device,such as a UE 115). In some examples, the beam direction associated withtransmissions along a single beam direction may be determined based atleast in in part on a signal that was transmitted in different beamdirections. For example, a UE 115 may receive one or more of the signalstransmitted by the base station 105 in different directions, and the UE115 may report to the base station 105 an indication of the signal itreceived with a highest signal quality, or an otherwise acceptablesignal quality. Although these techniques are described with referenceto signals transmitted in one or more directions by a base station 105,a UE 115 may employ similar techniques for transmitting signals multipletimes in different directions (e.g., for identifying a beam directionfor subsequent transmission or reception by the UE 115), or transmittinga signal in a single direction (e.g., for transmitting data to areceiving device).

A receiving device (e.g., a UE 115, which may be an example of a mmWreceiving device) may try multiple receive beams when receiving varioussignals from the base station 105, such as synchronization signals,reference signals, beam selection signals, or other control signals. Forexample, a receiving device may try multiple receive directions byreceiving via different antenna subarrays, by processing receivedsignals according to different antenna subarrays, by receiving accordingto different receive beamforming weight sets applied to signals receivedat a plurality of antenna elements of an antenna array, or by processingreceived signals according to different receive beamforming weight setsapplied to signals received at a plurality of antenna elements of anantenna array, any of which may be referred to as “listening” accordingto different receive beams or receive directions. In some examples, areceiving device may use a single receive beam to receive along a singlebeam direction (e.g., when receiving a data signal). The single receivebeam may be aligned in a beam direction determined based on listeningaccording to different receive beam directions (e.g., a beam directiondetermined to have a highest signal strength, highest signal-to-noiseratio, or otherwise acceptable signal quality based on listeningaccording to multiple beam directions).

In some cases, the antennas of a base station 105 or UE 115 may belocated within one or more antenna arrays, which may support MIMOoperations, or transmit or receive beamforming. For example, one or morebase station antennas or antenna arrays may be co-located at an antennaassembly, such as an antenna tower. In some cases, antennas or antennaarrays associated with a base station 105 may be located in diversegeographic locations. A base station 105 may have an antenna array witha number of rows and columns of antenna ports that the base station 105may use to support beamforming of communications with a UE 115.Likewise, a UE 115 may have one or more antenna arrays that may supportvarious MIMO or beamforming operations.

In some cases, wireless communication system 100 may be a packet-basednetwork that operate according to a layered protocol stack. In the userplane, communications at the bearer or Packet Data Convergence Protocol(PDCP) layer may be IP-based. A Radio Link Control (RLC) layer mayperform packet segmentation and reassembly to communicate over logicalchannels. A Medium Access Control (MAC) layer may perform priorityhandling and multiplexing of logical channels into transport channels.The MAC layer may also use hybrid automatic repeat request (HARQ) toprovide retransmission at the MAC layer to improve link efficiency. Inthe control plane, the Radio Resource Control (RRC) protocol layer mayprovide establishment, configuration, and maintenance of an RRCconnection between a UE 115 and a base station 105 or core network 130supporting radio bearers for user plane data. At the Physical layer,transport channels may be mapped to physical channels.

In some cases, UEs 115 and base stations 105 may support retransmissionsof data to increase the likelihood that data is received successfully.HARQ feedback is one technique of increasing the likelihood that data isreceived correctly over a communication link 125. HARQ may include acombination of error detection (e.g., using a cyclic redundancy check(CRC)), forward error correction (FEC), and retransmission (e.g.,automatic repeat request (ARQ)). HARQ may improve throughput at the MAClayer in poor radio conditions (e.g., signal-to-noise conditions). Insome cases, a wireless device may support same-slot HARQ feedback, wherethe device may provide HARQ feedback in a specific slot for datareceived in a previous symbol in the slot. In other cases, the devicemay provide HARQ feedback in a subsequent slot, or according to someother time interval.

Time intervals in LTE or NR may be expressed in multiples of a basictime unit, which may, for example, refer to a sampling period ofT_(s)=1/30,720,000 seconds. Time intervals of a communications resourcemay be organized according to radio frames each having a duration of 10milliseconds (ms), where the frame period may be expressed asT_(f)=307,200 T_(s). The radio frames may be identified by a systemframe number (SFN) ranging from 0 to 1023. Each frame may include 10subframes numbered from 0 to 9, and each subframe may have a duration of1 ms. A subframe may be further divided into 2 slots each having aduration of 0.5 ms, and each slot may contain 6 or 7 modulation symbolperiods (e.g., depending on the length of the cyclic prefix prepended toeach symbol period). Excluding the cyclic prefix, each symbol period maycontain 2048 sampling periods. In some cases, a subframe may be thesmallest scheduling unit of the wireless communication system 100, andmay be referred to as a transmission time interval (TTI). In othercases, a smallest scheduling unit of the wireless communication system100 may be shorter than a subframe or may be dynamically selected (e.g.,in bursts of shortened TTIs (sTTIs) or in selected component carriersusing sTTIs).

In some wireless communication systems, a slot may further be dividedinto multiple mini-slots containing one or more symbols. In someinstances, a symbol of a mini-slot or a mini-slot may be the smallestunit of scheduling. Each symbol may vary in duration depending on thesubcarrier spacing or frequency band of operation, for example. Further,some wireless communication systems may implement slot aggregation inwhich multiple slots or mini-slots are aggregated together and used forcommunication between a UE 115 and a base station 105.

The term “carrier” refers to a set of radio frequency spectrum resourceshaving a defined physical layer structure for supporting communicationsover a communication link 125. For example, a carrier of a communicationlink 125 may include a portion of a radio frequency spectrum band thatis operated according to physical layer channels for a given radioaccess technology. Each physical layer channel may carry user data,control information, or other signaling. A carrier may be associatedwith a pre-defined frequency channel (e.g., an evolved universal mobiletelecommunication system terrestrial radio access (E-UTRA) absoluteradio frequency channel number (EARFCN)), and may be positionedaccording to a channel raster for discovery by UEs 115. Carriers may bedownlink or uplink (e.g., in an FDD mode), or be configured to carrydownlink and uplink communications (e.g., in a TDD mode). In someexamples, signal waveforms transmitted over a carrier may be made up ofmultiple sub-carriers (e.g., using multi-carrier modulation (MCM)techniques such as orthogonal frequency division multiplexing (OFDM) ordiscrete Fourier transform spread OFDM (DFT-S-OFDM)).

The organizational structure of the carriers may be different fordifferent radio access technologies (e.g., LTE, LTE-A, LTE-A Pro, NR).For example, communications over a carrier may be organized according toTTIs or slots, each of which may include user data as well as controlinformation or signaling to support decoding the user data. A carriermay also include dedicated acquisition signaling (e.g., synchronizationsignals or system information, etc.) and control signaling thatcoordinates operation for the carrier. In some examples (e.g., in acarrier aggregation configuration), a carrier may also have acquisitionsignaling or control signaling that coordinates operations for othercarriers.

Physical channels may be multiplexed on a carrier according to varioustechniques. A physical control channel and a physical data channel maybe multiplexed on a downlink carrier, for example, using time divisionmultiplexing (TDM) techniques, frequency division multiplexing (FDM)techniques, or hybrid TDM-FDM techniques. In some examples, controlinformation transmitted in a physical control channel may be distributedbetween different control regions in a cascaded manner (e.g., between acommon control region or common search space and one or more UE-specificcontrol regions or UE-specific search spaces).

A carrier may be associated with a particular bandwidth of the radiofrequency spectrum, and in some examples the carrier bandwidth may bereferred to as a “system bandwidth” of the carrier or the wirelesscommunication system 100. For example, the carrier bandwidth may be oneof a number of predetermined or defined bandwidths for carriers of aparticular radio access technology (e.g., 1.4, 3, 5, 10, 15, 20, 40, or80 MHz). In some examples, each served UE 115 may be configured foroperating over portions or all of the carrier bandwidth. In otherexamples, some UEs 115 may be configured for operation using anarrowband protocol type that is associated with a predefined portion orrange (e.g., set of subcarriers or RBs) within a carrier (e.g.,“in-band” deployment of a narrowband protocol type).

In a system employing MCM techniques, a resource element may consist ofone symbol period (e.g., a duration of one modulation symbol) and onesubcarrier, where the symbol period and subcarrier spacing are inverselyrelated. The number of bits carried by each resource element may dependon the modulation scheme (e.g., the order of the modulation scheme).Thus, the more resource elements that a UE 115 receives and the higherthe order of the modulation scheme, the higher the data rate may be forthe UE 115. In MIMO systems, a wireless communications resource mayrefer to a combination of a radio frequency spectrum resource, a timeresource, and a spatial resource (e.g., spatial layers), and the use ofmultiple spatial layers may further increase the data rate forcommunications with a UE 115.

Devices of the wireless communication system 100 (e.g., base stations105 or UEs 115) may have a hardware configuration that supportscommunications over a particular carrier bandwidth, or may beconfigurable to support communications over one of a set of carrierbandwidths. In some examples, the wireless communication system 100 mayinclude base stations 105 and/or UEs 115 that support simultaneouscommunications via carriers associated with more than one differentcarrier bandwidth.

Wireless communication system 100 may support communication with a UE115 on multiple cells or carriers, a feature which may be referred to ascarrier aggregation or multi-carrier operation. A UE 115 may beconfigured with multiple downlink component carriers and one or moreuplink component carriers according to a carrier aggregationconfiguration. Carrier aggregation may be used with both FDD and TDDcomponent carriers.

In some cases, wireless communication system 100 may utilize enhancedcomponent carriers (eCCs). An eCC may be characterized by one or morefeatures including wider carrier or frequency channel bandwidth, shortersymbol duration, shorter TTI duration, or modified control channelconfiguration. In some cases, an eCC may be associated with a carrieraggregation configuration or a dual connectivity configuration (e.g.,when multiple serving cells have a suboptimal or non-ideal backhaullink). An eCC may also be configured for use in unlicensed spectrum orshared spectrum (e.g., where more than one operator is allowed to usethe spectrum). An eCC characterized by wide carrier bandwidth mayinclude one or more segments that may be utilized by UEs 115 that arenot capable of monitoring the whole carrier bandwidth or are otherwiseconfigured to use a limited carrier bandwidth (e.g., to conserve power).

In some cases, an eCC may utilize a different symbol duration than othercomponent carriers, which may include use of a reduced symbol durationas compared with symbol durations of the other component carriers. Ashorter symbol duration may be associated with increased spacing betweenadjacent subcarriers. A device, such as a UE 115 or base station 105,utilizing eCCs may transmit wideband signals (e.g., according tofrequency channel or carrier bandwidths of 20, 40, 60, 80 MHz, etc.) atreduced symbol durations (e.g., 16.67 microseconds). A TTI in eCC mayconsist of one or multiple symbol periods. In some cases, the TTIduration (that is, the number of symbol periods in a TTI) may bevariable.

Wireless communication system 100 may be an NR system that may utilizeany combination of licensed, shared, and unlicensed spectrum bands,among others. The flexibility of eCC symbol duration and subcarrierspacing may allow for the use of eCC across multiple spectrums. In someexamples, NR shared spectrum may increase spectrum utilization andspectral efficiency, specifically through dynamic vertical (e.g., acrossthe frequency domain) and horizontal (e.g., across the time domain)sharing of resources.

A UE 115 (e.g., a first UE 115) may establish a sidelink connection witha second UE 115, where the sidelink connection is associated with aplurality of flows. The UE 115 may determine, based on monitoring eachflow of the plurality of flows, a radio link status of the sidelinkconnection. The UE 115 may transmit, based on the determining, anon-access stratum layer message to the second UE 115 based at least onthe radio link status of the sidelink connection.

A UE 115 (e.g., the second UE 115) may establish a sidelink connectionwith a first UE 115, where the sidelink connection is associated with aplurality of flows. The UE 115 may receive a non-access stratum layermessage from the first UE 115 indicating a radio link status of thesidelink connection, where the non-access stratum layer message is basedon a status at the first UE of each flow of the plurality of flows. TheUE 115 may transmit a response message to the first UE 115 indicatingthat the radio link status of the sidelink connection with the first UE115 is active.

FIG. 2 illustrates an example of a wireless communication system 200that supports unicast link RLF detection and management in accordancewith aspects of the present disclosure. In some examples, wirelesscommunication system 200 may implement aspects of wireless communicationsystem 100. Wireless communication system 200 may include a base station205, UE 210, and a UE 215, which may be examples of the correspondingdevices described herein. In some aspects, the UE 210 and the UE 215 maybe communicating over a sidelink connection.

Wireless communication systems may use different interfaces to supportwireless communications between wireless devices. For example, a Uuinterface may be used to support wireless communications between basestation 205 and UE 210 and/or UE 215 over links 220 and/or 225,respectively. A PC5 interface may be used to support wirelesscommunications between UE 210 and UE 215. Each interface type is uniquein terms of configurations, requirements, etc. UE 210 and UE 215 mayestablish a sidelink connection over the PC5 interface, with thesidelink connection supporting multiple flows (with a first flow 230 anda second flow 235 being shown by way of example only). A flow broadlyrefers to data communicated over separate DRBs, data communicated havingdifferent QoS requirements, and the like. Some wireless communicationsystems are configured such that no AS layer signaling can be used forRLM/RLF. Some wireless communication systems may also be configured suchthat there is no receiver side lower layer indication supported for RLF.Accordingly, this may result in UEs communicating over a sidelinkchannel having no effective or efficient means of managing the sidelinkconnection. In some aspects, this may mean that the UEs unnecessarilydeclare RLF for the sidelink connection in the situation where the linkis still active, but may have degraded some degree. Unnecessarilydeclaring RLF means that considerable resources and time are wastedtearing down the sidelink connection and then rebuilding a new sidelinkconnection.

For example, and for an NR PC5 interface, a UE may establish a layer 2(L2) link (e.g., a sidelink connection) with the peer UE, with multipleassociated QoS flows/DRBs. The traffic for the NR PC5 interfaces may be,in some examples, periodic, e.g., there is no guarantee that there willalways be traffic for a particular flow. Some wireless communicationsystems may be configured such as there is no receiver sidein-sync/out-of-sync (e.g., active or inactive) indication provided toupper layers. Some wireless communication systems may instead, rely onHARQ feedback for the link maintenance. However, when there are multipleQoS flows (e.g., mapped to DRBs), each flow may use a differentconfiguration. Accordingly, a HARQ feedback from one QoS flow may notreflect problems for the other QoS flows. Moreover, due to aperiodictraffic, there may not be any HARQ feedbacks, which may further delaylink problem detection.

Accordingly, aspects of the described techniques may include performinga PC5-RRC-based aggregation of different RLF triggers (e.g., a loss ofdata communicated) from the DRBs (e.g., the plurality of flows) to avoidunnecessary RLF declarations. Aspects of the described techniquesutilize an efficient NAS layer keep alive mechanism to allow thedetection of the RLF at the receiver and transmitter side in a reliableand resource efficient manner. In some aspects, this may take intoaccount the unique QoS requirements of each flow (e.g., DRBs). In someaspects, this may include a PC5-RRC layer implemented at each UEaggregating different QoS flow/DRB information before triggering NASlayer actions for RLM/RLF signaling.

For example, the UE 210 and the UE 215 may establish a sidelinkconnection that includes, or is otherwise associated with, a pluralityof flows. In the example illustrated in wireless communication system200, the first flow 230 and the second flow 235 are illustrated.However, it is to be understood that the sidelink connection may havemore than two flows. Broadly, the UE 210 and the UE 215 may monitor eachflow in the sidelink connection to determine the radio link status forthe sidelink connection. Although the techniques described herein areprovided with reference to the UE 210 (e.g., a first UE), it is to beunderstood that the UE 215 (e.g., a second UE) may also be configured toimplement aspects of the described techniques. The UE 210 and UE 215 maygenerally exchange various messages at the AS layer and/or NAS layerbased on the radio link status of the sidelink connection.

For example, the UE 210 may monitor each flow configured for, orotherwise associated with, the sidelink connection to determine theradio link status. The monitoring may be based on an acknowledgmentfeedback information, e.g., determining whether acknowledgment ornegative-acknowledgment HARQ feedback messages are received and, if so,how many of each message type is received. For example, a thresholdnumber of negative-acknowledgement received within a time period maysignal that no data is being successfully communicated over the flow. Asanother example, failure to receive acknowledgement ornegative-acknowledgement messages for data transmitted over a flow maysignal that no data is being successfully communicated over the flow.

Additionally or alternatively, the monitoring may be based on a timerassociated with each flow (e.g., an inactivity timer). For example, theUE 210 may establish an inactivity timer for each flow to determine howmuch time has passed since data has been successfully communicated overthe flow. The value or threshold for the inactivity timer may be aconfigured value and/or may be determined by the UE 210 based onhistorical information for the flow (e.g., based on the observed natureof data communicated over the flow). The UE 210 may also establish theinactivity timer for the flow based on the QoS parameters associatedwith the flow, e.g., the PC5 five quality indicators (PQI), the datarate, maximum data burst rate (MDBV), averaging window, etc. Expirationof the inactivity timer may signal that no data is being successfullycommunicated over the flow.

Additionally or alternatively, the monitoring may be based on whetherdata communicated over a flow satisfies the QoS configuration for theflow. For example, a threshold amount of data communicated over a flowwithin a time period that fails to meet the QoS configuration for theflow may signal that no data is being successfully communicated over theflow.

The UE 210 may determine, based on the monitoring, whether there is datacommunicated successfully across each flow in the sidelink connection,e.g., based on the inactivity timer, HARQ feedback, etc. If the UE 210determines that there is no data being successfully communicated acrossthe first flow 230 (as one non-limiting example), it may trigger anerror indication (e.g., RLF trigger) to the PC5-RRC layer. Based on thetrigger, the PC5-RRC layer of the UE 210 may query the other activeflow(s) (e.g., the second flow 235 associated with the same destination,such as the L2 identifier (ID) or a L2 Link ID, with the same cast type)to determine whether there is data being successfully communicatedacross those flow(s). Based on the response to that query, the UE 210may take various steps.

In one situation, UE 210 may determine that there is no datasuccessfully communicated across the second flow 235 and/or any otherflows configured in the sidelink connection. In response, the UE 210 maytransmit a NAS layer message (e.g., a keep alive message or signaling)to the UE 215. Broadly, the NAS layer message may be configured toconvey a request for the UE 215 to confirm that the radio link status ofthe sidelink connection is active. In the situation where the UE 215receives the keep alive message, it may transmit a response message tothe UE 210 indicating that the radio link status of the sidelinkconnection is active. For example, the UE 215 may also monitor thedifferent flows configured for the sidelink connection to determine theradio link status from its perspective. If the response messageindicates that the sidelink connection is active, this may signal orotherwise convey an indication that the sidelink connection is active,but may have degraded to some degree (e.g., due to mobility) such thatthe data being communicated may not fully satisfy the QoS requirementassociated with the flow and/or that at least some of the data has beendropped.

Accordingly, the UE 210 and UE 215 may exchange one or more messages(e.g., at the NAS layer and/or at the AS layer) to reconfigure variousparameters of the sidelink connection. For example, UE 210 and the UE215 may select a new QoS requirement for a flow (e.g., the first flow230 in this example experiencing the loss of data communications) from aset of available or otherwise configured QoS configurations. That is,the UE 210 and UE 215 (and/or base station 205) may configure multipleQoS configurations for the sidelink connection. The UE 210 and UE 215may select from the QoS configurations for each flow. In the situationwhere communications over a flow degrade, UE 210 and UE 215 may select anew QoS configuration for the flow to improve data communications.

Additionally or alternatively, this may include modifying various NASlayer parameters associated with the QoS requirement for the flow aswell. For example, UE 210 and UE 215 may adjust the transmit powerlevel, may switch from negative-acknowledgement mode to acknowledgementmode for the flow, may adjust the Range for the flow, the Delay for theflow, may adjust the peak error rate (PER) for the flow, and the like.

Additionally or alternatively, UE 210 and the UE 215 may select a newconfiguration for the sidelink connection from a set of available orotherwise configured configurations for the sidelink connection. Thatis, UE 210 and UE 215 (and/or base station 205) may configure multipleconfigurations for the sidelink connection. UE 210 and UE 215 may selectfrom the different configurations for the sidelink connection to improvedata communications. Accordingly, this may enable the UE 210 and the UE215 to more efficiently manage each flow associated with the sidelinkconnection in a dynamic manner.

Additionally or alternatively, this may include modifying various ASlayer parameters associated with the QoS requirement for the flow aswell. For example, UE 210 and UE 215 may adjust the modulation andcoding scheme (MCS) for the flow, and the like.

In the situation where the UE 215 does not receive the keep alivemessage (and therefore cannot respond), the UE 210 may determine that noresponse message to the keep alive message was received from the UE 215.This may signal or otherwise convey an indication that the sidelinkconnection is inactive (e.g., that an RLF has occurred). Accordingly, UE210 may initiate an RLF recovery procedure where it tears down thesidelink connection with UE 215 and begins to establish a new sidelinkconnection. In some aspects, this may include the UE 210 tearing downthe L2 link (and releasing all associated resources) and reporting theerror to the base station 205 over link 220 (e.g., using the Uu RRCSidelinkUEInfo message). Alternatively, the NAS layer may choose toperform the RLF recovery by maintaining the security associations withUE 210 and performing another discovery for the same application layerID.

In another situation, UE 210 may determine that there is datacommunicated successfully across the second flow 235 and/or otherflow(s) in the sidelink connection. This may indicate that the radiolink status for the sidelink connection is a degraded radio link status,e.g., the sidelink connection performance has degraded to some degree,but is still suitable for wireless communications. In response, UE 210may transmit the NAS layer message (e.g., a keep alive message) to UE215. Broadly, the NAS layer message may be configured to convey arequest from the UE 215 confirming that the radio link status of thesidelink connection is active. In this situation, UE 215 may receive thekeep alive message and respond with a response message indicating thatthe radio link status of the sidelink connection is active. In thisexample, UE 210 may transmit another message (e.g., at the NAS layerand/or at the AS layer) to UE 215 reconfiguring various parameters ofthe sidelink connection, e.g., changing QoS parameters, selecting a newQoS configuration, selecting a new configuration for the sidelinkconnection, and the like.

In some aspects, transmission of the NAS layer message (e.g., the keepalive message) may be depended upon whether a previous keep alivemessage was transmitted within a threshold time period. That is, afterdetermining that the first flow 230 has no data being successfullycommunicated, UE 210 may determine when the last keep alive message wastransmitted to UE 215. If the last keep alive message was transmittedwithin the threshold time period, UE 210 may refrain from transmittinganother keep alive message at that time. Instead, UE 210 may againdetermine whether data is being successfully communicated successfullyacross the first flow 230 after the threshold time period and, if not,then transmit a second keep alive message if needed. In some examples,the NAS layer message may be a PC5 sidelink (PC5-S) message.

In some aspects, the described techniques may support QoS parameterbased RLF triggering and keep alive signaling cancellation based on theAS layer. That is, as the triggering of the error towards the NAS layercan be based on individual DRBs (e.g., flows), different configurationsfor the RLF triggers (e.g., the queries implemented at the PC5-RRClayer) can be configured based on the QoS of the flow. For example, foreach of the DRBs, there may be an associated PQI and a corresponding QoSparameters. Accordingly, the criteria for triggering the RLF error(e.g., the query) may be determined based on various factors. Forexample, based on the PQI, a guaranteed bit rate flow type may have anestimated traffic periodicity, e.g., considering the bit rate, anaveraging window, the typical packet size, etc. Accordingly, fordifferent DRBs, different inactivity timers could be used to trigger theRLF indication. From the transmitter side (e.g., UE 215), based on thetraffic pattern of the QoS flow (e.g., maximum data burst volume(MDBV)), UE 215 may determine that some bursty QoS flows should have ahigher RLC failure count before triggering the RLF indication. Thechannel busy ratio (CBR) may also be considered for such high burstflows.

With respect to the NAS layer keep alive signaling, this may consumeconsiderable radio resources. Therefore, in some examples the keep alivemessage may be triggered due to NAS layer indication. The keep alivetimer (e.g., a threshold time period) may be reset whenever the PC5-RRClayer has successful signaling. In some examples, a DRB layertransmission with successful HARQ can also cancel keep alive messaging.

Accordingly, aspects of the described techniques introduce keep alivesignaling in the PC5-S protocol. In the keep alive signaling management,this may take into account the AS layer indication (e.g., to reducewaste). The PC5-RRC layer may handle RLF trigger to the NAS layer basedon different DRB (e.g., flow) considerations. Aspects of the describedtechniques may also introduce PC5-RRC layer signaling to reconfigure theDRBs due to the RLF management. The UE configuration for the RLFdetection (e.g., the inactivity timer) may be based on the QoSinformation.

FIG. 3 illustrates an example of a process 300 that supports unicastlink RLF detection and management in accordance with aspects of thepresent disclosure. In some examples, process 300 may implement aspectsof wireless communication systems 100 and/or 200. Aspects of process 300may be implemented by a first UE 305 and a second UE 310, which may beexamples of corresponding devices described herein. In some aspects, thefirst UE 305 and the second UE 310 may be performing wirelesscommunications over a sidelink channel.

In some aspects, the first UE 305 may include a NAS layer 315 (e.g., aL3 or NAS protocol layer) and a PC5-RRC layer 320 (e.g., a L2 or ASprotocol layer). Similarly, the second UE 310 may include a NAS layer350 and a PC5-RRC layer 345. As discussed, the first UE 305 and thesecond UE 310 may have established a sidelink connection that includes aplurality of flows. Each flow may correspond to a different DRB and/orQoS requirement associated with data communications over the flow.Accordingly and in the example process 300, the first UE 305 may includea first DRB 325 and a second DRB 330. Similarly, the second UE 310 mayinclude a first DRB 340 and a second DRB 335. Thus, the first DRB 325 ofthe first UE 305 and first DRB 340 of the second UE 310 may correspondto, or otherwise be associated with, a first flow used for datacommunications over the sidelink channel. The second DRB 330 of thefirst UE 305 and the second DRB 335 of the second UE 310 may correspondto, or otherwise be associated with, a second flow used for datacommunications over the sidelink channel. As discussed above, the firstflow and second flow (as well as any other flows configured for thesidelink connection) may have separate configurations, e.g., QoSrequirements.

Accordingly and at 355, the first UE 305 and second UE 310 may beperforming wireless communications (e.g., communicating data) over thefirst flow (e.g., via the first DRB 325 of the first UE 305 and thefirst DRB 340 of the second UE 310) of the sidelink connection. The datacommunicated over the first flow may have an associated QoS requirement,e.g., a latency threshold, reliability threshold, throughputrequirement, etc.

Similarly and at 360, the first UE 305 and the second UE 310 may beperforming wireless communications over the second flow (e.g., via thesecond DRB 330 of the first UE 305 and the second DRB 335 of the secondUE 310) of the sidelink connection. The data communicated over thesecond flow may have an associated QoS requirement that is the same as,or different than, the QoS requirement of the first flow.

Generally, the first UE 305 and the second UE 310 may monitor datacommunicated over each flow configured for the sidelink connection.Based on the monitoring, each UE may determine the radio link status forthe sidelink connection. However, at 360 the first UE 305 may determinethat no data is being successfully communicated across the second flow(as indicated by the X). It is to be understood that the lack of datasuccessfully communicated over the second flow is provided by way ofnon-limiting example only and that the describe techniques may beimplemented with respect to any flow configured for the sidelinkconnection.

For example, the first UE 305 may determine that there is no data beingsuccessfully communicated over the second flow based on an inactivitytimer exceeding a threshold time/expiring, based on a lack ofacknowledgment messages received for data communicated over the secondflow, and the like. As discussed, the determination that no data isbeing successfully communicated (e.g., the RLF trigger) may beconfigured the same or differently for each flow. That is, the thresholdfor determining that no data is being successfully communicated over thesecond flow may be the same as or different from the threshold for thedetermination that no data is being successfully communicated over thefirst flow.

At 365 and for the first flow over DRB1, the first UE 305 may determinethat it has no data successfully communicated for a threshold time(e.g., based on an inactivity timer having a configured value) and/orthat it cannot send data over the first flow (e.g., based on HARQfeedback) and send an error indicator (e.g., RLF trigger) to the PC5-RRClayer 320 (e.g., the lack of data at 360 may trigger the error or RLFindication at 365). In some aspects, the error indication may be basedon data communicated over the second flow failing to satisfy the QoSrequirement associated with the second flow.

At 370 and in response to the error indication, the PC5-RRC layer 320may query the other flows (including the first flow over DRB1) of thesidelink connection to determine whether they have experienced a loss ofdata communications. For example, the PC5-RRC layer 320 may check theinactivity timer associated with the other flows, determine whether databeing communicated across the other flows is successful (e.g., based onHARQ feedback), determine whether data being communicated across theother flows satisfy the corresponding QoS requirements, and the like.Thus, the PC5-RRC layer 320 will query other active bearers (e.g., DRB1)having the same destination address (e.g., layer 2 ID having the samecast type). The next steps taken by the first UE 305 may depend on theresponse to the query at 370.

In a first option and at 375, the PC5-RRC layer 320 may determine thatthere is data being successfully communicated across the other flows(such as the first flow) of the sidelink channel. For example, thePC5-RRC layer 320 may determine that the inactivity timer associatedwith the other flows may not have reached a threshold/expired, that datais being communicated and acknowledged across the other flows, and thelike. In some aspects, the response to the query may also includemeasurement info, e.g., CQI, RSRP, etc., for the associated flow.

In response to determining that data is being successfully communicatedacross the other flows, the PC5-RRC layer 320 may take several steps.One-step may include canceling the trigger received at 365. That is, thedetermination that data is being communicated across the other flows mayindicate that the sidelink connection has not experienced an RLF and,therefore, avoid unnecessarily triggering an RLF recovery procedureand/or transmission of the NAS layer message. In another example and at380, the first UE 305 may initiate PC5-RRC signaling to the second UE310 to change to a different configuration. That is, the first UE 305may transmit a message to the second UE 310 (at the NAS and/or AS layer)reconfiguring one or more parameters of the sidelink connection. Thismay include adjusting an MCS, a power level, range, etc., to improvecompliance with the associated QoS requirements. This may includeselecting a new configuration for the QoS configuration and/or a newconfiguration for the sidelink channel.

In some aspects, this may include triggering the NAS layer 315 to updatethe QoS flow configuration. In some aspects, the PC5-RRC layer 320 ofthe first UE 305 and the PC5-RRC layer 345 of the second UE 310 may havea set of QoS configurations configured for the flow, and select a newQoS configuration without informing the NAS layer 315 of the first UE305 and/or NAS layer 350 of the second UE 310.

In a second option and at 385, the query with regards to the data beingsuccessfully communicated across the other flows may return an errorindication. For example, the error indication may also be associatedwith an inactivity timers for the other flows reaching correspondingthresholds (e.g., expiring), failure to satisfy QoS requirementsconfigured for each flow, a lack of HARQ feedback for data communicatedacross the flows, and the like.

In response and at 390, the PC5-RRC layer 320 may indicate an error(e.g., an RLF trigger) to the NAS layer 315. At 390, this may triggerthe NAS layer 315 to initiate (e.g., transmit) keep alive signaling withthe NAS layer 350 of the second UE 310. The keep alive signaling (e.g.,a NAS layer message) transmitted at the NAS layer 315 may be sent overone or more of the flows and/or an SRB (e.g., to improve reliability andfeedback from the second UE 310). In some aspects, the error indicationmay also carry or convey information identifying a cause for the errorindication.

If the second UE 310 receives the keep alive signaling at 395 andresponds, this may trigger the PC5-RRC layer 320 of the first UE 305 andthe PC5-RRC layer 345 of the second UE 310 to exchange various messagesto reconfigure parameters associated with the flows of the sidelinkconnection. For example, they may renegotiate the DRBs and/or the QoSflow configurations for any impacted flow of the sidelink connection.

However, if the NAS layer signaling returns an error (e.g., the first UE305 does not receive a response to the keep alive signaling), the firstUE 305 may declare an RLF and initiate an RLF recovery procedure, e.g.,tear down the L2 link (e.g., the sidelink connection) and release allassociated resources and report the error to its base station.Alternatively, the NAS layer 315 of the first UE 305 and the NAS layer350 of the second UE 310 may choose to perform the RLF recovery bykeeping (e.g., maintaining) the security associations and having anotherdiscovery for the same application layer ID.

Accordingly, process 300 illustrates various techniques that can be usedat the AS layer (e.g., the PC5-RRC layers) and/or the NAS layers of thefirst UE 305 and the second UE 310 in the event that an RLF is detectedon at least one flow (e.g., at least one DRB) of the sidelinkconnection. This may avoid the situation where an RLF is unnecessarilydeclared, which would utilize substantial over the air resources toestablish a new sidelink connection.

FIG. 4 illustrates an example of a process 400 that supports unicastlink RLF detection and management in accordance with aspects of thepresent disclosure. In some examples, process 400 may implement aspectsof wireless communication systems 100 and/or 200 and/or process 300.Aspects of process 400 may be implemented by a first UE 405 and a secondUE 410, which may be examples of corresponding devices described herein.In some aspects, the first UE 405 and the second UE 410 may beperforming wireless communications over a sidelink channel.

In some aspects, the first UE 405 may include a NAS layer 415 (e.g., aL3 or NAS protocol layer) and a PC5-RRC layer 420 (e.g., a L2 or ASprotocol layer). Similarly, the second UE 410 may include a NAS layer450 and a PC5-RRC layer 445. As discussed, the first UE 405 and thesecond UE 410 may have established a sidelink connection that includes aplurality of flows. Each flow may correspond to a different DRB and/orQoS requirement associated with data communications. Accordingly and inthe example process 400, the first UE 405 may include a first DRB 425and a second DRB 430. Similarly, the second UE 410 may include a firstDRB 440 and a second DRB 435. Thus, the first DRB 425 of the first UE405 and first DRB 440 of the second UE 410 may correspond to, orotherwise be associated with, a first flow used for data communicationsover the sidelink channel. The second DRB 430 of the first UE 405 andthe second DRB 435 of the second UE 410 may correspond to, or otherwisebe associated with, a second flow used for data communications over thesidelink channel. As discussed above, the first flow and second flow (aswell as any other flows configured for the sidelink connection) may haveseparate configurations, e.g., QoS requirements.

Accordingly, at 455, the first UE 405 and second UE 410 may beperforming wireless communications (e.g., communicating data) over thefirst flow (e.g., via the first DRB 425 of the first UE 405 and thefirst DRB 440 of the second UE 410) of the sidelink connection. The datacommunicated over the first flow may have an associated QoS requirement,e.g., a latency threshold, reliability threshold, throughputrequirement, etc.

Similarly, at 460, the first UE 405 and the second UE 410 may beperforming wireless communications over the second flow (e.g., via thesecond DRB 430 of the first UE 405 and the second DRB 435 of the secondUE 410) of the sidelink connection. The data communicated over thesecond flow may have an associated QoS requirement that is the same asor different than the QoS requirement of the first flow.

Generally, the first UE 405 and the second UE 410 may monitor datacommunicated over each flow configured for the sidelink connection.Based on the monitoring, each UE may determine the radio link status forthe sidelink connection. However, at 460 the first UE 405 may determinethat no data is being successfully communicated across the second flow(as indicated by the X). It is to be understood that the lack of datasuccessfully communicated over the second flow is provided by way ofnon-limiting example only and that the describe techniques may beimplemented with respect to any flow configured for the sidelinkconnection.

For example, the first UE 405 may determine that there is no data beingsuccessfully communicated over the second flow based on an inactivitytimer exceeding a threshold time/expiring, based on a lack ofacknowledgment messages received for data communicated over the firstflow, based on data communicated over the first flow failing to satisfythe associated QoS requirements, and the like. As discussed, thedetermination that no data is being successfully communicated (e.g., theRLF trigger) may be configured the same or differently for each flow.That is, the threshold for determining that no data is beingsuccessfully communicated over the first flow may be the same as ordifferent from the threshold for the determination that no data is beingsuccessfully communicated over the second flow.

At 465, the NAS layer 415 of the first UE 405 and the NAS layer 450 ofthe second UE 410 may optionally exchange a NAS layer message (e.g., akeep alive message). For example, the keep alive message may have beenexchanged due to the first UE 405 and/or the second UE 410 previouslydetecting a loss of data communications over one of the flows of thesidelink connection, according to a periodic schedule, and the like.

At 470 and for the first flow over DRB1, the first UE 405 may determinethat it has no data for a threshold time (e.g., based on an inactivitytimer having a configured value expiring) and/or that it cannot senddata over the first flow (e.g., based on HARQ feedback) and send anerror indication (e.g., RLF trigger) to the NAS layer 415 (e.g., thelack of data at 460 may trigger the RLF indication at 470). In someaspects, the error indication may be based on data communicated over thesecond flow failing to satisfy the QoS requirement associated with thatflow, based on a lack of acknowledgment messaging received for datacommunicated over the flow, based on the inactivity timer expiring orreaching a threshold time, and the like.

That is, in some aspects the NAS layer 415 (e.g., PC5-S, V2X layer) mayhandle the error from the AS layer based on the keep alive messagingstatus. When data communicated over the first flow experiencesinterruption, this triggers the error indication to the NAS layer 415.In some aspects, the error indication may carry or convey informationdifferentiating the cause for the trigger, e.g., due to inactivity timerexpiration, due to consecutive transmission errors (e.g., based on HARQfeedback and/or RLC error count reaching a threshold), and the like.

Based on the error indication and the triggering cause code, the NASlayer 415 may determine the appropriate action to take. As one exampleand for the situation where the inactivity timer expires, the NAS layer415 may wait for the next keep alive period to send the next keep alivemessage (e.g., in the situation where keep alive signaling isperiodically exchanged between the NAS layer 415 of the first UE 405 andthe NAS layer 450 of the second UE 410). For example, the NAS layer 415may determine that keep alive signaling was exchanged at 465 and/or thatanother scheduled keep alive message is approaching, and therefore waituntil the next scheduled keep alive message.

As another example and for the situation where the trigger is due totransmission errors, the NAS layer 415 may trigger the keep alivesignaling immediately. Accordingly and at 475, the NAS layer 415 of thefirst UE 405 may transmit a keep alive message to the NAS layer 450 ofthe second UE 410. In some aspects of this example, the NAS layer 415may also determine whether previous NAS layer message (e.g., the keepalive signaling) was transmitted within a threshold time period and, ifso, may provide feedback to the second flow to wait until the next keepalive signaling is scheduled.

If the second UE 410 receives the keep alive signaling at 475 andresponds, this may trigger PC5-RRC layer 420 of the first UE 405 andPC5-RRC layer 445 of the second UE 410 to exchange various messages toreconfigure parameters associated with the flows of the sidelinkconnection. For example, they may renegotiate the DRBs and/or the QoSflow configurations for any impacted flow of the sidelink connection.

However, if the NAS layer signaling returns an error (e.g., the first UE405 does not receive a response to the keep alive signaling), the firstUE 405 may declare an RLF and initiate an RLF recovery procedure, e.g.,to tear down the L2 link (e.g., the sidelink connection) and release allassociated resources and report the error to its base station.Alternatively, the NAS layer 415 of the first UE 405 and the NAS layer450 of the second UE 410 may choose to perform the RLF recovery bykeeping (e.g., maintaining) the security associations and having anotherdiscovery for the same application layer ID.

Accordingly, process 400 illustrates various techniques that can be usedat the AS layer (e.g., the PC5-RRC layers) and/or the NAS layer of thefirst UE 405 and the second UE 410 in the event that an RLF is detectedon at least one flow (e.g., at least one DRB) of the sidelinkconnection. This may avoid the situation where an RLF is unnecessarilydeclared, which would utilize substantial over the air resources toestablish a new sidelink connection.

FIG. 5 illustrates an example of a process 500 that supports unicastlink RLF detection and management in accordance with aspects of thepresent disclosure. In some examples, process 500 may implement aspectsof wireless communication systems 100 and/or 200 and/or processes 300and/or 400. Aspects of process 500 may be implemented by a first UE 505and/or a second UE 510, which may be examples of corresponding devicesdescribed herein. In some aspects, the first UE 505 and the second UE510 may be performing wireless communications over a sidelinkconnection.

At 515, the first UE 505 and the second UE 510 may establish a sidelinkconnection. The sidelink connection may be established over a PC5interface and may include multiple flows. Generally, each flow maycorrespond to a stream of data being communicated over the sidelinkconnection via a particular DRB, according to a specific QoSrequirement, and the like.

At 520, the first UE 505 may determine the radio link status of thesidelink connection. For example, the first UE 505 may monitor datacommunicated over each flow of the sidelink connection to determine theradio link status. Although not shown, it is to be understood that thesecond UE 510 may also monitor communications over the flows of thesidelink connection to determine the radio link status from itsperspective.

At 525, the first UE 505 may transmit (and the second UE 510 mayreceive) a NAS layer message based on the radio link status of thesidelink connection. In some aspects, the NAS layer message may includekeep alive signaling to determine whether the sidelink connection isactive. In some aspects, the NAS layer message (and/or AS layermessaging) may include information reconfiguring various QoS parametersof the flows.

In some aspects, this may include the first UE 505 determining that nodata is being successfully communicated across a first flow (and/or anyother flow(s)) of the plurality of flows of the sidelink connection.Accordingly, the first UE 505 may transmit the NAS layer message to thesecond UE 510, with the message including keep alive signalingrequesting confirmation from the second UE 510 that the radio linkstatus of the sidelink connection is active. If the second UE 510transmits a response message (and the first UE 505 receives theresponse) indicating that the radio link status of the sidelinkconnection is active, the first UE 505 may transmit (and the second UE510 may receive) a second message (e.g., at the NAS layer and/or at theAS layer) reconfiguring one or more parameters of the sidelinkconnection. For example, they may reconfigure the sidelink connectionfrom a first configuration to a second configuration from a set ofavailable configurations (e.g., from a set of available configurations).As another example they may reconfigure one or more QoS parametersconfigured for the sidelink connection. In some aspects, the first UE505 may determine that it did not receive a response message from thesecond UE 510. Based on the lack of response, the first UE 505 maydetermine that the radio link status of the sidelink connection is RLF.Accordingly, the first UE 505 may perform an RLF recovery procedure toestablish a second sidelink connection with the second UE 510.

In some aspects, the first UE 505 may determine that data beingcommunicated across the first flow fails to satisfy the correspondingQoS requirement associated with the first flow. In some aspects, thismay include the first UE 505 determining that no data has beensuccessfully communicated across the first flow for a threshold timeperiod associated with the first flow (e.g., based on an inactivitytimer).

In some aspects, this may include the first UE 505 determining that nodata is successfully communicated across a first flow, but that data isbeing successfully communicated across a second flow of the sidelinkconnection (and/or any other flow of the sidelink connection).Accordingly, the first UE 505 may determine that the radio link statusof the sidelink connection is a degraded radio link status. Therefore,the first UE 505 may transmit (and the second UE 510 may receive) asecond message (e.g., at the NAS layer and/or AS layer) reconfiguringone or more parameters of the sidelink connection, e.g., selecting a newconfiguration for the sidelink connection, reconfiguring QoS parameters,and the like.

In some aspects, the first UE 505 may determine that the NAS layermessage was transmitted within a threshold time period and, therefore,refrain from transmitting a second NAS layer message. In some aspects,the NAS layer message may include a PC5-S message.

FIG. 6 shows a block diagram 600 of a device 605 that supports unicastlink RLF detection and management in accordance with aspects of thepresent disclosure. The device 605 may be an example of aspects of a UE115 as described herein. The device 605 may include a receiver 610, acommunications manager 615, and a transmitter 620. The device 605 mayalso include a processor. Each of these components may be incommunication with one another (e.g., via one or more buses).

The receiver 610 may receive information such as packets, user data, orcontrol information associated with various information channels (e.g.,control channels, data channels, and information related to unicast linkRLF detection and management, etc.). Information may be passed on toother components of the device 605. The receiver 610 may be an exampleof aspects of the transceiver 920 described with reference to FIG. 9.The receiver 610 may utilize a single antenna or a set of antennas.

The communications manager 615 may establish a sidelink connection witha second UE, where the sidelink connection is associated with a set offlows, determine, based on monitoring each flow of the set of flows, aradio link status of the sidelink connection, and transmit, based on thedetermining, a non-access stratum layer message to the second UE basedat least on the radio link status of the sidelink connection.

The communications manager 615 may also establish a sidelink connectionwith a first UE, where the sidelink connection is associated with a setof flows, receive a non-access stratum layer message from the first UEindicating a radio link status of the sidelink connection, where thenon-access stratum layer message is based on a status at the first UE ofeach flow of the set of flows, and transmit a response message to thefirst UE indicating that the radio link status of the sidelinkconnection with the first UE is active. The communications manager 615may be an example of aspects of the communications manager 910 describedherein.

The actions performed by the communications manager 615 as describedherein may be implemented to realize one or more potential advantages.One implementation may allow a UE 115 to save power and increase batterylife by determining the radio link status of the sidelink connection.Another implementation may provide improved quality and reliability ofservice at the UE 115, as sidelink connections may be enhanced.

The communications manager 615, or its sub-components, may beimplemented in hardware, code (e.g., software or firmware) executed by aprocessor, or any combination thereof. If implemented in code executedby a processor, the functions of the communications manager 615, or itssub-components may be executed by a general-purpose processor, a digitalsignal processor (DSP), an application-specific integrated circuit(ASIC), a field-programmable gate array (FPGA) or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed in the present disclosure.

The communications manager 615, or its sub-components, may be physicallylocated at various positions, including being distributed such thatportions of functions are implemented at different physical locations byone or more physical components. In some examples, the communicationsmanager 615, or its sub-components, may be a separate and distinctcomponent in accordance with various aspects of the present disclosure.In some examples, the communications manager 615, or its sub-components,may be combined with one or more other hardware components, includingbut not limited to an input/output (I/O) component, a transceiver, anetwork server, another computing device, one or more other componentsdescribed in the present disclosure, or a combination thereof inaccordance with various aspects of the present disclosure.

The transmitter 620 may transmit signals generated by other componentsof the device 605. In some examples, the transmitter 620 may becollocated with a receiver 610 in a transceiver module. For example, thetransmitter 620 may be an example of aspects of the transceiver 920described with reference to FIG. 9. The transmitter 620 may utilize asingle antenna or a set of antennas.

FIG. 7 shows a block diagram 700 of a device 705 that supports unicastlink RLF detection and management in accordance with aspects of thepresent disclosure. The device 705 may be an example of aspects of adevice 605, or a UE 115 as described herein. The device 705 may includea receiver 710, a communications manager 715, and a transmitter 735. Thedevice 705 may also include a processor. Each of these components may bein communication with one another (e.g., via one or more buses).

The receiver 710 may receive information such as packets, user data, orcontrol information associated with various information channels (e.g.,control channels, data channels, and information related to unicast linkRLF detection and management, etc.). Information may be passed on toother components of the device 705. The receiver 710 may be an exampleof aspects of the transceiver 920 described with reference to FIG. 9.The receiver 710 may utilize a single antenna or a set of antennas.

The communications manager 715 may be an example of aspects of thecommunications manager 615 as described herein. The communicationsmanager 715 may include a connection manager 720, a radio link statusmanager 725, and a NAS layer message manager 730. The communicationsmanager 715 may be an example of aspects of the communications manager910 described herein.

The connection manager 720 may establish a sidelink connection with asecond UE, where the sidelink connection is associated with a set offlows.

The radio link status manager 725 may determine, based on monitoringeach flow of the set of flows, a radio link status of the sidelinkconnection.

The NAS layer message manager 730 may transmit, based on thedetermining, a non-access stratum layer message to the second UE basedat least on the radio link status of the sidelink connection.

The connection manager 720 may establish a sidelink connection with afirst UE, where the sidelink connection is associated with a set offlows.

The NAS layer message manager 730 may receive a non-access stratum layermessage from the first UE indicating a radio link status of the sidelinkconnection, where the non-access stratum layer message is based on astatus at the first UE of each flow of the set of flows and transmit aresponse message to the first UE indicating that the radio link statusof the sidelink connection with the first UE is active.

The transmitter 735 may transmit signals generated by other componentsof the device 705. In some examples, the transmitter 735 may becollocated with a receiver 710 in a transceiver module. For example, thetransmitter 735 may be an example of aspects of the transceiver 920described with reference to FIG. 9. The transmitter 735 may utilize asingle antenna or a set of antennas.

FIG. 8 shows a block diagram 800 of a communications manager 805 thatsupports unicast link RLF detection and management in accordance withaspects of the present disclosure. The communications manager 805 may bean example of aspects of a communications manager 615, a communicationsmanager 715, or a communications manager 910 described herein. Thecommunications manager 805 may include a connection manager 810, a radiolink status manager 815, a NAS layer message manager 820, a RLF manager825, a degraded link manager 830, a NAS message timing manager 835, anda QoS manager 840. Each of these modules may communicate, directly orindirectly, with one another (e.g., via one or more buses).

The connection manager 810 may establish a sidelink connection with asecond UE, where the sidelink connection is associated with a set offlows. In some examples, the connection manager 810 may establish asidelink connection with a first UE, where the sidelink connection isassociated with a set of flows.

The radio link status manager 815 may determine, based on monitoringeach flow of the set of flows, a radio link status of the sidelinkconnection.

The NAS layer message manager 820 may transmit, based on thedetermining, a non-access stratum layer message to the second UE basedat least on the radio link status of the sidelink connection. In someexamples, the NAS layer message manager 820 may receive a non-accessstratum layer message from the first UE indicating a radio link statusof the sidelink connection, where the non-access stratum layer messageis based on a status at the first UE of each flow of the set of flows.In some examples, the NAS layer message manager 820 may transmit aresponse message to the first UE indicating that the radio link statusof the sidelink connection with the first UE is active. In some cases,the non-access stratum layer message includes a PC5-S message. In somecases, the non-access stratum layer message includes a keep-alivemessage requesting confirmation from the second UE that the radio linkstatus of the sidelink connection is active.

The RLF manager 825 may determine that no data is communicated across afirst flow of the set of flows. In some examples, transmitting thenon-access stratum layer message to the second UE, where the non-accessstratum layer message includes a keep-alive message requestingconfirmation from the second UE that the radio link status of thesidelink connection is active. In some examples, the RLF manager 825 mayreceive a response message from the second UE indicating that the radiolink status of the sidelink connection with the second UE is active. Insome examples, the RLF manager 825 may transmit, based on the responsemessage, a second message to the second UE reconfiguring one or moreparameters of the sidelink connection. In some examples, the RLF manager825 may reconfigure the sidelink connection from a first configurationto a second configuration from a set of available configurationsconfigured for the sidelink connection. In some examples, the RLFmanager 825 may reconfigure one or more quality of service parametersconfigured for the sidelink connection. In some examples, determining,based on a lack of a response message from the second UE, that the radiolink status of the sidelink connection with the second UE includes aradio link failure.

In some examples, the RLF manager 825 may perform, based on the radiolink failure, a radio link failure recovery procedure to establish asecond sidelink connection with the second UE. In some examples, the RLFmanager 825 may determine that data communicated across the first flowfails to satisfy a quality of service requirement associated with thefirst flow. In some examples, the RLF manager 825 may determine that nodata has been communicated across the first flow for a threshold timeperiod associated with the first flow.

The degraded link manager 830 may determine that no data is communicatedacross a first flow of the set of flows. In some examples, the degradedlink manager 830 may determine that data is communicated across a secondflow of the set of flows. In some examples, determining that the radiolink status of the sidelink connection includes a degraded radio linkstatus.

In some examples, the degraded link manager 830 may transmit a secondmessage to the second UE reconfiguring one or more parameters of thesidelink connection. In some examples, the degraded link manager 830 mayreconfigure the sidelink connection from a first configuration to asecond configuration from a set of available configurations configuredfor the sidelink connection. In some examples, the degraded link manager830 may reconfigure one or more quality of service parameters configuredfor the sidelink connection. In some examples, the degraded link manager830 may receive, based on the response message, a second message fromthe first UE reconfiguring one or more parameters of the sidelinkconnection. In some examples, the degraded link manager 830 mayreconfigure the one or more parameters of the sidelink connection basedat least on part on the second message.

In some examples, the degraded link manager 830 may reconfigure thesidelink connection from a first configuration to a second configurationfrom a set of available configurations configured for the sidelinkconnection. In some examples, the degraded link manager 830 mayreconfigure one or more quality of service parameters configured for thesidelink connection.

The NAS message timing manager 835 may determine that the non-accessstratum layer message was transmitted within a threshold time period. Insome examples, the NAS message timing manager 835 may refrain fromtransmitting a second non-access stratum layer message to the second UEbased at least on the non-access stratum layer message.

The QoS manager 840 may determine, for each flow of the set of flows,whether data communicated across each flow satisfies a quality ofservice requirement configured for the flow.

FIG. 9 shows a diagram of a system 900 including a device 905 thatsupports unicast link RLF detection and management in accordance withaspects of the present disclosure. The device 905 may be an example ofor include the components of device 605, device 705, or a UE 115 asdescribed herein. The device 905 may include components forbi-directional voice and data communications including components fortransmitting and receiving communications, including a communicationsmanager 910, an I/O controller 915, a transceiver 920, an antenna 925,memory 930, and a processor 940. These components may be in electroniccommunication via one or more buses (e.g., bus 945).

The communications manager 910 may establish a sidelink connection witha second UE, where the sidelink connection is associated with a set offlows, determine, based on monitoring each flow of the set of flows, aradio link status of the sidelink connection, and transmit, based on thedetermining, a non-access stratum layer message to the second UE basedat least on the radio link status of the sidelink connection.

The communications manager 910 may also establish a sidelink connectionwith a first UE, where the sidelink connection is associated with a setof flows, receive a non-access stratum layer message from the first UEindicating a radio link status of the sidelink connection, where thenon-access stratum layer message is based on a status at the first UE ofeach flow of the set of flows, and transmit a response message to thefirst UE indicating that the radio link status of the sidelinkconnection with the first UE is active.

The I/O controller 915 may manage input and output signals for thedevice 905. The I/O controller 915 may also manage peripherals notintegrated into the device 905. In some cases, the I/O controller 915may represent a physical connection or port to an external peripheral.In some cases, the I/O controller 915 may utilize an operating systemsuch as iOS®, ANDROID®, MS-DOS®, MS-WINDOWS®, OS/2®, UNIX®, LINUX®, oranother known operating system. In other cases, the I/O controller 915may represent or interact with a modem, a keyboard, a mouse, atouchscreen, or a similar device. In some cases, the I/O controller 915may be implemented as part of a processor. In some cases, a user mayinteract with the device 905 via the I/O controller 915 or via hardwarecomponents controlled by the I/O controller 915.

The transceiver 920 may communicate bi-directionally, via one or moreantennas, wired, or wireless links as described herein. For example, thetransceiver 920 may represent a wireless transceiver and may communicatebi-directionally with another wireless transceiver. The transceiver 920may also include a modem to modulate the packets and provide themodulated packets to the antennas for transmission, and to demodulatepackets received from the antennas.

In some cases, the wireless device may include a single antenna 925.However, in some cases the device may have more than one antenna 925,which may be capable of concurrently transmitting or receiving multiplewireless transmissions.

The memory 930 may include random-access memory (RAM) and read-onlymemory (ROM). The memory 930 may store computer-readable,computer-executable code 935 including instructions that, when executed,cause the processor to perform various functions described herein. Insome cases, the memory 930 may contain, among other things, a basic I/Osystem (BIOS) which may control basic hardware or software operationsuch as the interaction with peripheral components or devices.

The processor 940 may include an intelligent hardware device, (e.g., ageneral-purpose processor, a DSP, a CPU, a microcontroller, an ASIC, anFPGA, a programmable logic device, a discrete gate or transistor logiccomponent, a discrete hardware component, or any combination thereof).In some cases, the processor 940 may be configured to operate a memoryarray using a memory controller. In other cases, a memory controller maybe integrated into the processor 940. The processor 940 may beconfigured to execute computer-readable instructions stored in a memory(e.g., the memory 930) to cause the device 905 to perform variousfunctions (e.g., functions or tasks supporting unicast link RLFdetection and management).

The code 935 may include instructions to implement aspects of thepresent disclosure, including instructions to support wirelesscommunications. The code 935 may be stored in a non-transitorycomputer-readable medium such as system memory or other type of memory.In some cases, the code 935 may not be directly executable by theprocessor 940 but may cause a computer (e.g., when compiled andexecuted) to perform functions described herein.

The actions performed by the processor 940, memory 930, I/O controller915, communications manager 910, transceiver 920, and antenna 925 asdescribed herein may be implemented to realize one or more potentialadvantages. One implementation may allow the device 905 to save powerand increase battery life by reconfiguring the sidelink connection fromthe first configuration to the second configuration from the set ofavailable configurations configured for the sidelink connection. Anotherimplementation may provide improved reliability and user experience atthe device 905 through the reduction of signaling overhead.

FIG. 10 shows a flowchart illustrating a method 1000 that supportsunicast link RLF detection and management in accordance with aspects ofthe present disclosure. The operations of method 1000 may be implementedby a UE 115 or its components as described herein. For example, theoperations of method 1000 may be performed by a communications manageras described with reference to FIGS. 6 through 9. In some examples, a UEmay execute a set of instructions to control the functional elements ofthe UE to perform the functions described herein. Additionally oralternatively, a UE may perform aspects of the functions describedherein using special-purpose hardware.

At 1005, the UE may establish a sidelink connection with a second UE,where the sidelink connection is associated with a set of flows. Theoperations of 1005 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1005 may beperformed by a connection manager as described with reference to FIGS. 6through 9.

At 1010, the UE may determine, based on monitoring each flow of the setof flows, a radio link status of the sidelink connection. The operationsof 1010 may be performed according to the methods described herein. Insome examples, aspects of the operations of 1010 may be performed by aradio link status manager as described with reference to FIGS. 6 through9.

At 1015, the UE may transmit, based on the determining, a non-accessstratum layer message to the second UE based at least on the radio linkstatus of the sidelink connection. The operations of 1015 may beperformed according to the methods described herein. In some examples,aspects of the operations of 1015 may be performed by a NAS layermessage manager as described with reference to FIGS. 6 through 9.

FIG. 11 shows a flowchart illustrating a method 1100 that supportsunicast link RLF detection and management in accordance with aspects ofthe present disclosure. The operations of method 1100 may be implementedby a UE 115 or its components as described herein. For example, theoperations of method 1100 may be performed by a communications manageras described with reference to FIGS. 6 through 9. In some examples, a UEmay execute a set of instructions to control the functional elements ofthe UE to perform the functions described herein. Additionally oralternatively, a UE may perform aspects of the functions describedherein using special-purpose hardware.

At 1105, the UE may establish a sidelink connection with a second UE,where the sidelink connection is associated with a set of flows. Theoperations of 1105 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1105 may beperformed by a connection manager as described with reference to FIGS. 6through 9.

At 1110, the UE may determine, based on monitoring each flow of the setof flows, a radio link status of the sidelink connection. The operationsof 1110 may be performed according to the methods described herein. Insome examples, aspects of the operations of 1110 may be performed by aradio link status manager as described with reference to FIGS. 6 through9.

At 1115, the UE may determine that no data is communicated across afirst flow of the set of flows. The operations of 1115 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 1115 may be performed by a RLF manager as describedwith reference to FIGS. 6 through 9.

At 1120, the UE may transmit, based on the determining, a non-accessstratum layer message to the second UE based at least on the radio linkstatus of the sidelink connection. The operations of 1120 may beperformed according to the methods described herein. In some examples,aspects of the operations of 1120 may be performed by a NAS layermessage manager as described with reference to FIGS. 6 through 9.

At 1125, the UE may transmit the non-access stratum layer message to thesecond UE, where the non-access stratum layer message includes akeep-alive message requesting confirmation from the second UE that theradio link status of the sidelink connection is active. The operationsof 1125 may be performed according to the methods described herein. Insome examples, aspects of the operations of 1125 may be performed by aRLF manager as described with reference to FIGS. 6 through 9.

FIG. 12 shows a flowchart illustrating a method 1200 that supportsunicast link RLF detection and management in accordance with aspects ofthe present disclosure. The operations of method 1200 may be implementedby a UE 115 or its components as described herein. For example, theoperations of method 1200 may be performed by a communications manageras described with reference to FIGS. 6 through 9. In some examples, a UEmay execute a set of instructions to control the functional elements ofthe UE to perform the functions described herein. Additionally oralternatively, a UE may perform aspects of the functions describedherein using special-purpose hardware.

At 1205, the UE may establish a sidelink connection with a second UE,where the sidelink connection is associated with a set of flows. Theoperations of 1205 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1205 may beperformed by a connection manager as described with reference to FIGS. 6through 9.

At 1210, the UE may determine, based on monitoring each flow of the setof flows, a radio link status of the sidelink connection. The operationsof 1210 may be performed according to the methods described herein. Insome examples, aspects of the operations of 1210 may be performed by aradio link status manager as described with reference to FIGS. 6 through9.

At 1215, the UE may transmit, based on the determining, a non-accessstratum layer message to the second UE based at least on the radio linkstatus of the sidelink connection. The operations of 1215 may beperformed according to the methods described herein. In some examples,aspects of the operations of 1215 may be performed by a NAS layermessage manager as described with reference to FIGS. 6 through 9.

At 1220, the UE may determine that no data is communicated across afirst flow of the set of flows. The operations of 1220 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 1220 may be performed by a degraded link manager asdescribed with reference to FIGS. 6 through 9.

At 1225, the UE may determine that data is communicated across a secondflow of the set of flows. The operations of 1225 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 1225 may be performed by a degraded link manager asdescribed with reference to FIGS. 6 through 9.

At 1230, the UE may determine that the radio link status of the sidelinkconnection includes a degraded radio link status. The operations of 1230may be performed according to the methods described herein. In someexamples, aspects of the operations of 1230 may be performed by adegraded link manager as described with reference to FIGS. 6 through 9.

At 1235, the UE may transmit a second message to the second UEreconfiguring one or more parameters of the sidelink connection. Theoperations of 1235 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1235 may beperformed by a degraded link manager as described with reference toFIGS. 6 through 9.

FIG. 13 shows a flowchart illustrating a method 1300 that supportsunicast link RLF detection and management in accordance with aspects ofthe present disclosure. The operations of method 1300 may be implementedby a UE 115 or its components as described herein. For example, theoperations of method 1300 may be performed by a communications manageras described with reference to FIGS. 6 through 9. In some examples, a UEmay execute a set of instructions to control the functional elements ofthe UE to perform the functions described herein. Additionally oralternatively, a UE may perform aspects of the functions describedherein using special-purpose hardware.

At 1305, the UE may establish a sidelink connection with a first UE,where the sidelink connection is associated with a set of flows. Theoperations of 1305 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1305 may beperformed by a connection manager as described with reference to FIGS. 6through 9.

At 1310, the UE may receive a non-access stratum layer message from thefirst UE indicating a radio link status of the sidelink connection,where the non-access stratum layer message is based on a status at thefirst UE of each flow of the set of flows. The operations of 1310 may beperformed according to the methods described herein. In some examples,aspects of the operations of 1310 may be performed by a NAS layermessage manager as described with reference to FIGS. 6 through 9.

At 1315, the UE may transmit a response message to the first UEindicating that the radio link status of the sidelink connection withthe first UE is active. The operations of 1315 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 1315 may be performed by a NAS layer message manageras described with reference to FIGS. 6 through 9.

FIG. 14 shows a flowchart illustrating a method 1400 that supportsunicast link RLF detection and management in accordance with aspects ofthe present disclosure. The operations of method 1400 may be implementedby a UE 115 or its components as described herein. For example, theoperations of method 1400 may be performed by a communications manageras described with reference to FIGS. 6 through 9. In some examples, a UEmay execute a set of instructions to control the functional elements ofthe UE to perform the functions described herein. Additionally oralternatively, a UE may perform aspects of the functions describedherein using special-purpose hardware.

At 1405, the UE may establish a sidelink connection with a first UE,where the sidelink connection is associated with a set of flows. Theoperations of 1405 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1405 may beperformed by a connection manager as described with reference to FIGS. 6through 9.

At 1410, the UE may receive a non-access stratum layer message from thefirst UE indicating a radio link status of the sidelink connection,where the non-access stratum layer message is based on a status at thefirst UE of each flow of the set of flows. The operations of 1410 may beperformed according to the methods described herein. In some examples,aspects of the operations of 1410 may be performed by a NAS layermessage manager as described with reference to FIGS. 6 through 9.

At 1415, the UE may transmit a response message to the first UEindicating that the radio link status of the sidelink connection withthe first UE is active. The operations of 1415 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 1415 may be performed by a NAS layer message manageras described with reference to FIGS. 6 through 9.

At 1420, the UE may receive, based on the response message, a secondmessage from the first UE reconfiguring one or more parameters of thesidelink connection. The operations of 1420 may be performed accordingto the methods described herein. In some examples, aspects of theoperations of 1420 may be performed by a degraded link manager asdescribed with reference to FIGS. 6 through 9.

At 1425, the UE may reconfigure the one or more parameters of thesidelink connection based at least on part on the second message. Theoperations of 1425 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1425 may beperformed by a degraded link manager as described with reference toFIGS. 6 through 9.

The following provides an overview of aspects of the present disclosure:

Aspect 1: A method for wireless communication at a first UE, comprising:establishing a sidelink connection with a second UE, wherein thesidelink connection is associated with a plurality of flows;determining, based at least in part on monitoring each flow of theplurality of flows, a radio link status of the sidelink connection; andtransmitting, based on the determining, a non-access stratum layermessage to the second UE based at least on the radio link status of thesidelink connection.

Aspect 2: The method of aspect 1, further comprising: determining thatno data is communicated across a first flow of the plurality of flows;and transmitting the non-access stratum layer message to the second UE,wherein the non-access stratum layer message comprises a keep-alivemessage requesting confirmation from the second UE that the radio linkstatus of the sidelink connection is active.

Aspect 3: The method of aspect 2, wherein determining that no data iscommunicated comprises: determining that data communicated across thefirst flow fails to satisfy a quality of service requirement associatedwith the first flow.

Aspect 4: The method of any of aspects 2 through 3, wherein determiningthat no data is communicated comprises: determining that no data hasbeen communicated across the first flow for a threshold time periodassociated with the first flow.

Aspect 5: The method of any of aspects 1 through 4, further comprising:receiving a response message from the second UE indicating that theradio link status of the sidelink connection with the second UE isactive; and transmitting, based at least in part on the responsemessage, a second message to the second UE reconfiguring one or moreparameters of the sidelink connection.

Aspect 6: The method of aspect 5, wherein reconfiguring one or moreparameters of the sidelink connection comprises: reconfiguring thesidelink connection from a first configuration to a second configurationfrom a set of available configurations configured for the sidelinkconnection.

Aspect 7: The method of any of aspects 5 through 6, whereinreconfiguring one or more parameters of the sidelink connectioncomprises: reconfiguring one or more quality of service parametersconfigured for the sidelink connection.

Aspect 8: The method of any of aspects 1 through 7, further comprising:determining, based at least in part on a lack of a response message fromthe second UE, that the radio link status of the sidelink connectionwith the second UE comprises a radio link failure; and performing, basedat least in part on the radio link failure, a radio link failurerecovery procedure to establish a second sidelink connection with thesecond UE.

Aspect 9: The method of any of aspects 1 through 8, further comprising:determining that no data is communicated across a first flow of theplurality of flows; determining that data is communicated across asecond flow of the plurality of flows; determining that the radio linkstatus of the sidelink connection comprises a degraded radio linkstatus; and transmitting a second message to the second UE reconfiguringone or more parameters of the sidelink connection.

Aspect 10: The method of aspect 9, wherein reconfiguring one or moreparameters of the sidelink connection comprises: reconfiguring thesidelink connection from a first configuration to a second configurationfrom a set of available configurations configured for the sidelinkconnection.

Aspect 11: The method of any of aspects 9 through 10, whereinreconfiguring one or more parameters of the sidelink connectioncomprises: reconfiguring one or more quality of service parametersconfigured for the sidelink connection.

Aspect 12: The method of any of aspects 1 through 11, furthercomprising: determining that the non-access stratum layer message wastransmitted within a threshold time period; and refraining fromtransmitting a second non-access stratum layer message to the second UEbased at least on the non-access stratum layer message.

Aspect 13: The method of any of aspects 1 through 12, whereindetermining the radio link status of the sidelink connection comprises:determining, for each flow of the plurality of flows, whether datacommunicated across each flow satisfies a quality of service requirementconfigured for the flow.

Aspect 14: The method of any of aspects 1 through 13, wherein thenon-access stratum layer message comprises a PC5 sidelink (PC5-S)message.

Aspect 15: A method for wireless communication at a second UE,comprising: establishing a sidelink connection with a first UE, whereinthe sidelink connection is associated with a plurality of flows;receiving a non-access stratum layer message from the first UEindicating a radio link status of the sidelink connection, wherein thenon-access stratum layer message is based at least in part on a statusat the first UE of each flow of the plurality of flows; and transmittinga response message to the first UE indicating that the radio link statusof the sidelink connection with the first UE is active.

Aspect 16: The method of aspect 15, further comprising: receiving, basedat least in part on the response message, a second message from thefirst UE reconfiguring one or more parameters of the sidelinkconnection; and reconfiguring the one or more parameters of the sidelinkconnection based at least on part on the second message.

Aspect 17: The method of aspect 16, wherein reconfiguring one or moreparameters of the sidelink connection comprises: reconfiguring thesidelink connection from a first configuration to a second configurationfrom a set of available configurations configured for the sidelinkconnection.

Aspect 18: The method of any of aspects 16 through 17, whereinreconfiguring one or more parameters of the sidelink connectioncomprises: reconfiguring one or more quality of service parametersconfigured for the sidelink connection.

Aspect 19: The method of any of aspects 15 through 18, wherein thenon-access stratum layer message comprises a keep-alive messagerequesting confirmation from the second UE that the radio link status ofthe sidelink connection is active.

Aspect 20: An apparatus for wireless communication at a first UE,comprising a processor; memory coupled with the processor; andinstructions stored in the memory and executable by the processor tocause the apparatus to perform a method of any of aspects 1 through 14.

Aspect 21: An apparatus for wireless communication at a first UE,comprising at least one means for performing a method of any of aspects1 through 14.

Aspect 22: A non-transitory computer-readable medium storing code forwireless communication at a first UE, the code comprising instructionsexecutable by a processor to perform a method of any of aspects 1through 14.

Aspect 23: An apparatus for wireless communication at a second UE,comprising a processor; memory coupled with the processor; andinstructions stored in the memory and executable by the processor tocause the apparatus to perform a method of any of aspects 15 through 19.

Aspect 24: An apparatus for wireless communication at a second UE,comprising at least one means for performing a method of any of aspects15 through 19.

Aspect 25: A non-transitory computer-readable medium storing code forwireless communication at a second UE, the code comprising instructionsexecutable by a processor to perform a method of any of aspects 15through 19.

It should be noted that the methods described herein describe possibleimplementations, and that the operations and the steps may be rearrangedor otherwise modified and that other implementations are possible.Further, aspects from two or more of the methods may be combined.

Techniques described herein may be used for various wirelesscommunication systems such as code division multiple access (CDMA), timedivision multiple access (TDMA), frequency division multiple access(FDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), and other systems.A CDMA system may implement a radio technology such as CDMA2000,Universal Terrestrial Radio Access (UTRA), etc. CDMA2000 covers IS-2000,IS-95, and IS-856 standards. IS-2000 Releases may be commonly referredto as CDMA2000 1×, 1×, etc. IS-856 (TIA-856) is commonly referred to asCDMA2000 1×EV-DO, High Rate Packet Data (HRPD), etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. A TDMA system mayimplement a radio technology such as Global System for MobileCommunications (GSM).

An OFDMA system may implement a radio technology such as Ultra MobileBroadband (UMB), Evolved UTRA (E-UTRA), Institute of Electrical andElectronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802.20, Flash-OFDM, etc. UTRA and E-UTRA are part of Universal MobileTelecommunications System (UMTS). LTE, LTE-A, and LTE-A Pro are releasesof UMTS that use E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A, LTE-A Pro, NR,and GSM are described in documents from the organization named “3rdGeneration Partnership Project” (3GPP). CDMA2000 and UMB are describedin documents from an organization named “3rd Generation PartnershipProject 2” (3GPP2). The techniques described herein may be used for thesystems and radio technologies mentioned herein as well as other systemsand radio technologies. While aspects of an LTE, LTE-A, LTE-A Pro, or NRsystem may be described for purposes of example, and LTE, LTE-A, LTE-APro, or NR terminology may be used in much of the description, thetechniques described herein are applicable beyond LTE, LTE-A, LTE-A Pro,or NR applications.

A macro cell generally covers a relatively large geographic area (e.g.,several kilometers in radius) and may allow unrestricted access by UEswith service subscriptions with the network provider. A small cell maybe associated with a lower-powered base station, as compared with amacro cell, and a small cell may operate in the same or different (e.g.,licensed, unlicensed, etc.) frequency bands as macro cells. Small cellsmay include pico cells, femto cells, and micro cells according tovarious examples. A pico cell, for example, may cover a small geographicarea and may allow unrestricted access by UEs with service subscriptionswith the network provider. A femto cell may also cover a smallgeographic area (e.g., a home) and may provide restricted access by UEshaving an association with the femto cell (e.g., UEs in a closedsubscriber group (CSG), UEs for users in the home, and the like). An eNBfor a macro cell may be referred to as a macro eNB. An eNB for a smallcell may be referred to as a small cell eNB, a pico eNB, a femto eNB, ora home eNB. An eNB may support one or multiple (e.g., two, three, four,and the like) cells, and may also support communications using one ormultiple component carriers.

The wireless communication systems described herein may supportsynchronous or asynchronous operation. For synchronous operation, thebase stations may have similar frame timing, and transmissions fromdifferent base stations may be approximately aligned in time. Forasynchronous operation, the base stations may have different frametiming, and transmissions from different base stations may not bealigned in time. The techniques described herein may be used for eithersynchronous or asynchronous operations.

Information and signals described herein may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the description may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof.

The various illustrative blocks and modules described in connection withthe disclosure herein may be implemented or performed with ageneral-purpose processor, a DSP, an ASIC, an FPGA, or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described herein. A general-purpose processor may be amicroprocessor, but in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices(e.g., a combination of a DSP and a microprocessor, multiplemicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration).

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope of the disclosure and appended claims. For example, due to thenature of software, functions described herein can be implemented usingsoftware executed by a processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations.

Computer-readable media includes both non-transitory computer storagemedia and communication media including any medium that facilitatestransfer of a computer program from one place to another. Anon-transitory storage medium may be any available medium that can beaccessed by a general purpose or special purpose computer. By way ofexample, and not limitation, non-transitory computer-readable media mayinclude RAM, ROM, electrically erasable programmable ROM (EEPROM), flashmemory, compact disk (CD) ROM or other optical disk storage, magneticdisk storage or other magnetic storage devices, or any othernon-transitory medium that can be used to carry or store desired programcode means in the form of instructions or data structures and that canbe accessed by a general-purpose or special-purpose computer, or ageneral-purpose or special-purpose processor. Also, any connection isproperly termed a computer-readable medium. For example, if the softwareis transmitted from a website, server, or other remote source using acoaxial cable, fiber optic cable, twisted pair, digital subscriber line(DSL), or wireless technologies such as infrared, radio, and microwave,then the coaxial cable, fiber optic cable, twisted pair, DSL, orwireless technologies such as infrared, radio, and microwave areincluded in the definition of medium. Disk and disc, as used herein,include CD, laser disc, optical disc, digital versatile disc (DVD),floppy disk and Blu-ray disc where disks usually reproduce datamagnetically, while discs reproduce data optically with lasers.Combinations of the above are also included within the scope ofcomputer-readable media.

As used herein, including in the claims, “or” as used in a list of items(e.g., a list of items prefaced by a phrase such as “at least one of” or“one or more of”) indicates an inclusive list such that, for example, alist of at least one of A, B, or C means A or B or C or AB or AC or BCor ABC (i.e., A and B and C). Also, as used herein, the phrase “basedon” shall not be construed as a reference to a closed set of conditions.For example, an exemplary step that is described as “based on conditionA” may be based on both a condition A and a condition B withoutdeparting from the scope of the present disclosure. In other words, asused herein, the phrase “based on” shall be construed in the same manneras the phrase “based at least in part on.”

In the appended figures, similar components or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If just the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label, or othersubsequent reference label.

The description set forth herein, in connection with the appendeddrawings, describes example configurations and does not represent allthe examples that may be implemented or that are within the scope of theclaims. The term “exemplary” used herein means “serving as an example,instance, or illustration,” and not “preferred” or “advantageous overother examples.” The detailed description includes specific details forthe purpose of providing an understanding of the described techniques.These techniques, however, may be practiced without these specificdetails. In some instances, well-known structures and devices are shownin block diagram form in order to avoid obscuring the concepts of thedescribed examples.

The description herein is provided to enable a person skilled in the artto make or use the disclosure. Various modifications to the disclosurewill be readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other variations withoutdeparting from the scope of the disclosure. Thus, the disclosure is notlimited to the examples and designs described herein, but is to beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method for wireless communication at a firstuser equipment (UE), comprising: establishing a sidelink connection witha second UE, wherein the sidelink connection is associated with aplurality of flows; determining, based at least in part on monitoringeach flow of the plurality of flows, a radio link status of the sidelinkconnection; and transmitting, based on the determining, a non-accessstratum layer message to the second UE based at least on the radio linkstatus of the sidelink connection.
 2. The method of claim 1, furthercomprising: determining that no data is communicated across a first flowof the plurality of flows; and transmitting the non-access stratum layermessage to the second UE, wherein the non-access stratum layer messagecomprises a keep-alive message requesting confirmation from the secondUE that the radio link status of the sidelink connection is active. 3.The method of claim 1, further comprising: receiving a response messagefrom the second UE indicating that the radio link status of the sidelinkconnection with the second UE is active; and transmitting, based atleast in part on the response message, a second message to the second UEreconfiguring one or more parameters of the sidelink connection.
 4. Themethod of claim 3, wherein reconfiguring one or more parameters of thesidelink connection comprises: reconfiguring the sidelink connectionfrom a first configuration to a second configuration from a set ofavailable configurations configured for the sidelink connection.
 5. Themethod of claim 3, wherein reconfiguring one or more parameters of thesidelink connection comprises: reconfiguring one or more quality ofservice parameters configured for the sidelink connection.
 6. The methodof claim 1, further comprising: determining, based at least in part on alack of a response message from the second UE, that the radio linkstatus of the sidelink connection with the second UE comprises a radiolink failure; and performing, based at least in part on the radio linkfailure, a radio link failure recovery procedure to establish a secondsidelink connection with the second UE.
 7. The method of claim 2,wherein determining that no data is communicated comprises: determiningthat data communicated across the first flow fails to satisfy a qualityof service requirement associated with the first flow.
 8. The method ofclaim 2, wherein determining that no data is communicated comprises:determining that no data has been communicated across the first flow fora threshold time period associated with the first flow.
 9. The method ofclaim 1, further comprising: determining that no data is communicatedacross a first flow of the plurality of flows; determining that data iscommunicated across a second flow of the plurality of flows; determiningthat the radio link status of the sidelink connection comprises adegraded radio link status; and transmitting a second message to thesecond UE reconfiguring one or more parameters of the sidelinkconnection.
 10. The method of claim 9, wherein reconfiguring one or moreparameters of the sidelink connection comprises: reconfiguring thesidelink connection from a first configuration to a second configurationfrom a set of available configurations configured for the sidelinkconnection.
 11. The method of claim 9, wherein reconfiguring one or moreparameters of the sidelink connection comprises: reconfiguring one ormore quality of service parameters configured for the sidelinkconnection.
 12. The method of claim 1, further comprising: determiningthat the non-access stratum layer message was transmitted within athreshold time period; and refraining from transmitting a secondnon-access stratum layer message to the second UE based at least on thenon-access stratum layer message.
 13. The method of claim 1, whereindetermining the radio link status of the sidelink connection comprises:determining, for each flow of the plurality of flows, whether datacommunicated across each flow satisfies a quality of service requirementconfigured for the flow.
 14. The method of claim 1, wherein thenon-access stratum layer message comprises a PC5 sidelink (PC5-S)message.
 15. A method for wireless communication at a second userequipment (UE), comprising: establishing a sidelink connection with afirst UE, wherein the sidelink connection is associated with a pluralityof flows; receiving a non-access stratum layer message from the first UEindicating a radio link status of the sidelink connection, wherein thenon-access stratum layer message is based at least in part on a statusat the first UE of each flow of the plurality of flows; and transmittinga response message to the first UE indicating that the radio link statusof the sidelink connection with the first UE is active.
 16. The methodof claim 15, further comprising: receiving, based at least in part onthe response message, a second message from the first UE reconfiguringone or more parameters of the sidelink connection; and reconfiguring theone or more parameters of the sidelink connection based at least on parton the second message.
 17. The method of claim 16, wherein reconfiguringone or more parameters of the sidelink connection comprises:reconfiguring the sidelink connection from a first configuration to asecond configuration from a set of available configurations configuredfor the sidelink connection.
 18. The method of claim 16, whereinreconfiguring one or more parameters of the sidelink connectioncomprises: reconfiguring one or more quality of service parametersconfigured for the sidelink connection.
 19. The method of claim 15,wherein the non-access stratum layer message comprises a keep-alivemessage requesting confirmation from the second UE that the radio linkstatus of the sidelink connection is active.
 20. An apparatus forwireless communication at a first user equipment (UE), comprising: aprocessor, memory coupled with the processor; and instructions stored inthe memory and executable by the processor to cause the apparatus to:establish a sidelink connection with a second UE, wherein the sidelinkconnection is associated with a plurality of flows; determine, based atleast in part on monitoring each flow of the plurality of flows, a radiolink status of the sidelink connection; and transmit, based on thedetermining, a non-access stratum layer message to the second UE basedat least on the radio link status of the sidelink connection.
 21. Theapparatus of claim 20, wherein the instructions are further executableby the processor to cause the apparatus to: determine that no data iscommunicated across a first flow of the plurality of flows; and transmitthe non-access stratum layer message to the second UE, wherein thenon-access stratum layer message comprises a keep-alive messagerequesting confirmation from the second UE that the radio link status ofthe sidelink connection is active.
 22. The apparatus of claim 21,wherein the instructions are further executable by the processor tocause the apparatus to: receive a response message from the second UEindicating that the radio link status of the sidelink connection withthe second UE is active; and transmit, based at least in part on theresponse message, a second message to the second UE reconfiguring one ormore parameters of the sidelink connection.
 23. The apparatus of claim22, wherein the instructions to reconfigure one or more parameters ofthe sidelink connection are executable by the processor to cause theapparatus to: reconfigure the sidelink connection from a firstconfiguration to a second configuration from a set of availableconfigurations configured for the sidelink connection.
 24. The apparatusof claim 22, wherein the instructions to reconfigure one or moreparameters of the sidelink connection are executable by the processor tocause the apparatus to: reconfigure one or more quality of serviceparameters configured for the sidelink connection.
 25. The apparatus ofclaim 21, wherein the instructions are further executable by theprocessor to cause the apparatus to: determine, based at least in parton a lack of a response message from the second UE, that the radio linkstatus of the sidelink connection with the second UE comprises a radiolink failure; and perform, based at least in part on the radio linkfailure, a radio link failure recovery procedure to establish a secondsidelink connection with the second UE.
 26. The apparatus of claim 21,wherein the instructions to determine that no data is communicated areexecutable by the processor to cause the apparatus to: determine thatdata communicated across the first flow fails to satisfy a quality ofservice requirement associated with the first flow.
 27. The apparatus ofclaim 21, wherein the instructions to determine that no data iscommunicated are executable by the processor to cause the apparatus to:determine that no data has been communicated across the first flow for athreshold time period associated with the first flow.
 28. The apparatusof claim 20, wherein the instructions are further executable by theprocessor to cause the apparatus to: determine that no data iscommunicated across a first flow of the plurality of flows; determinethat data is communicated across a second flow of the plurality offlows; determine that the radio link status of the sidelink connectioncomprises a degraded radio link status; and transmit a second message tothe second UE reconfiguring one or more parameters of the sidelinkconnection.
 29. The apparatus of claim 28, wherein the instructions toreconfigure one or more parameters of the sidelink connection areexecutable by the processor to cause the apparatus to: reconfigure thesidelink connection from a first configuration to a second configurationfrom a set of available configurations configured for the sidelinkconnection.
 30. The apparatus of claim 28, wherein the instructions toreconfigure one or more parameters of the sidelink connection areexecutable by the processor to cause the apparatus to: reconfigure oneor more quality of service parameters configured for the sidelinkconnection.
 31. The apparatus of claim 20, wherein the instructions arefurther executable by the processor to cause the apparatus to: determinethat the non-access stratum layer message was transmitted within athreshold time period; and refrain from transmitting a second non-accessstratum layer message to the second UE based at least on the non-accessstratum layer message.
 32. The apparatus of claim 20, wherein theinstructions to determine the radio link status of the sidelinkconnection are executable by the processor to cause the apparatus to:determine, for each flow of the plurality of flows, whether datacommunicated across each flow satisfies a quality of service requirementconfigured for the flow.
 33. The apparatus of claim 20, wherein thenon-access stratum layer message comprises a PC5 sidelink (PC5-S)message.
 34. An apparatus for wireless communication at a second userequipment (UE), comprising: a processor, memory coupled with theprocessor; and instructions stored in the memory and executable by theprocessor to cause the apparatus to: establish a sidelink connectionwith a first UE, wherein the sidelink connection is associated with aplurality of flows; receive a non-access stratum layer message from thefirst UE indicating a radio link status of the sidelink connection,wherein the non-access stratum layer message is based at least in parton a status at the first UE of each flow of the plurality of flows; andtransmit a response message to the first UE indicating that the radiolink status of the sidelink connection with the first UE is active. 35.The apparatus of claim 34, wherein the instructions are furtherexecutable by the processor to cause the apparatus to: receive, based atleast in part on the response message, a second message from the firstUE reconfiguring one or more parameters of the sidelink connection; andreconfigure the one or more parameters of the sidelink connection basedat least on part on the second message.
 36. The apparatus of claim 35,wherein the instructions to reconfigure one or more parameters of thesidelink connection are executable by the processor to cause theapparatus to: reconfigure the sidelink connection from a firstconfiguration to a second configuration from a set of availableconfigurations configured for the sidelink connection.
 37. The apparatusof claim 35, wherein the instructions to reconfigure one or moreparameters of the sidelink connection are executable by the processor tocause the apparatus to: reconfigure one or more quality of serviceparameters configured for the sidelink connection.
 38. The apparatus ofclaim 34, wherein the non-access stratum layer message comprises akeep-alive message requesting confirmation from the second UE that theradio link status of the sidelink connection is active.